Nelson Bay may have been waterlogged under the 150mm of rain it was pelted with in the 24-hours to 9am Thursday - and another 50mm in the next eight hours to 5pm - but the Port Stephens town is forecast to get as much as 200mm today.

Meanwhile Newcastle is predicted to receive as much as 120mm of rain.

The BoM said "persistent and heavy rainfall" would affect the Lower Hunter, Sydney and the Illawarra from today into the weekend.

"A Flood Watch has been issued for the Mid North Coast, Hunter and parts of Sydney," the bureau said in a statement. "Residents and visitors in affected areas are urged to closely monitor warnings and updates over the coming days."

State Emergency Service crews at Newcastle, Lake Macquarie and Port Stephens were run ragged yesterday, with 88 calls for help in the 24-hours to 2.30pm - 30 of those came in the two hours from midday.

Part of Foreshore Road at Salamander Bay was washed away by rainwater rapids and SES teams were called to help stop leaks at NSW Ambulance's Charlestown base, St Peter's Anglican Church at Hamilton and several daycare centres across the region.

Hunter SES spokesperson Alex Varley said the teams were getting ready for a weekend that was expected to be "long" for emergency responders.

"Forecasts indicate that conditions will likely continue to deteriorate over the next 24-hours as the system moves," she told the Newcastle Herald on Thursday afternoon.

Several roads went under water yesterday, including parts of King Street at Warners Bay, the Inner City Bypass at North Lambton and the Pacific Highway at Heatherbrae, Ferodale and Gateshead.

Ms Varley urged people to take care on the roads and not to drive into floodwater under any circumstances.

"Don't take the risk, it's not worth it," she said.

"It puts families at risk and our local volunteers at risk as well."

The University of Newcastle announced its Callaghan and Ourimbah campuses would be closed from Friday until Sunday evening to "ensure the safety of our students, staff and visitors to the campus". The city and Sydney campuses will open as normal.

The state's transport authority has replaced trains with buses between Maitland and Scone-Dungog because of flooding on the Hunter line.

"Passengers are advised to delay their trip or allow plenty of extra travel time, listen to announcements and check information displays for service updates," a Transport Management Centre spokesperson said.

The Friday Night Feast, set to be held at The Station in Newcastle, has been cancelled, as has the Old Fashioned Picnic Day, which was planned for Tocal Homestead this Sunday

Port Stephens Council has closed Samurai Beach and the campground, saying it was "unsafe for vehicles to travel through this area".

Hunter Water issued a statement on social media saying the heavy rain was putting pressure on the storm water system.

"It's advised people avoid swimming in local beaches and waterways for two days after heavy rain events, like the one were experiencing," the agency said. "Heavy rainfall is putting pressure on our system, with widespread downpours across our area of operations. Heavy rain events lead to stormwater and debris infiltrating the wastewater system and can cause overflows."

Marine Rescue NSW urged people to delay plans to go onto the water until the weather eases.

"In these conditions, staying at home is the safest option. It's definitely not worth risking your life for a few fish," commissioner Stacey Tannos said.

Update - 5.30pm:

The State Emergency Service is bracing as weather conditions are expected to deteriorate further on Friday, with the Bureau of Meteorology forecasting "possibly life-threatening" heavy rainfall and damaging winds for the Mid North Coast and parts of the Hunter.

The BoM is predicting as much as 120mm to 200mm of rain at Nelson Bay on Friday, with 80mm to 150mm tipped for Newcastle.

"Forecasts indicate that conditions will likely continue to deteriorate over the next 24 hours as the system moves," Hunter SES spokesperson Alex Varley told the Newcastle Herald.

The BoM says "persistent and heavy rainfall" will affect the Lower Hunter, Sydney and the Illawarra from Friday into the weekend.

"A Flood Watch has been issued for the Mid North Coast, Hunter and parts of Sydney," a statement from BoM said on Thursday afternoon.

"Residents and visitors in affected areas are urged to closely monitor warnings and updates over coming days."

The University of Newcastle has announced its Callaghan and Ourimbah campuses will be closed from Friday until Sunday evening ahead of conditions worsening, to "ensure the safety of our students, staff and visitors to the campus".

The city and Sydney campuses will remain open.

Meanwhile, the state's transport authority has replaced trains with buses between Maitland and Scone-Dungog due to flooding on the Hunter Line.

"Passengers are advised to delay their trip or allow plenty of extra travel time, listen to announcements and check information displays for service updates," a Transport Management Centre spokesperson said in a statement.

The Friday Night Feast, set to be held at The Station in Newcastle, has been cancelled due to the weather forecast, as has the Old Fashioned Picnic Day which was planned for Tocal Homestead this Sunday.

Update - 3pm:

A massive 150mm of rain has fallen at Nelson Bay in the 24-hours to 2.30pm on Thursday, according to Bureau of Meteorology figures.

Bungwahl, near Karuah, has had 142mm for the period, Merewether has recorded 57mm and Williamtown has registered 44mm.

Meanwhile, Port Stephens Council has closed Samurai Beach and the campground, saying in a statement it was "unsafe for vehicles to travel through this area". The council also closed several roads in the district.

State Emergency Service spokesperson David Rankine told the Newcastle Herald SES crews in the Newcastle, Lake Macquarie and Port Stephens areas had received 88 requests for assistance in the 24-hours to 2.30pm.

Of that total, Mr Rankine said, 30 calls came between 12pm and 2pm.

He said most of the calls related to leaking roofs and requests for sandbags.

Teams have responded to leaks at the Charlestown NSW Ambulance station, St Peters Anglican Church at Hamilton and several childcare centres.

Mr Rankine urged people to be patient after calling for help because the crews were prioritising jobs but they would respond to calls as soon as possible.

"The vast majority have been storm-related jobs. We've seen a lot of people with either leaking roofs or water rising into garages," he said.

Hunter SES spokesperson Alex Varley said the crews were getting ready for a weekend that was expected to be "long" for emergency responders.

She urged people to take care on the roads and not to drive into floodwater under any circumstances.

"Don't take the risk, it's not worth it," she said.

"It puts families at risk and our local volunteers at risk as well."

Port Stephens council said via Twitter that several roads had been closed. These were:

- Kirrang Drive, Medowie

- Foreshore Drive, Corlette

- Riverside Drive, Karuah Tomaree Headland - Shoal Bay Road to boat ramp

- Dowling Street, Nelson Bay

-Horace Street, Shoal Bay

-Marine Drive, Fingal Bay - near Barry Park

- Old Main Road, Anna Bay - tree over road

- Stockton Pond, Nelson Bay- near Moorooba Crescent

- Teramby Road, Nelson Bay - land slip onto road

Update - 1pm:

A flood warning from the Bureau of Meteorology has been issued for parts of the Hunter and other coastal areas.

The warning said the possible flooding was expected from Thursday into the weekend.

The bureau said Wollombi Brook and the Lower Hunter River faced possible minor-to-moderate flooding, while the Myall, Paterson and Williams rivers faced potential minor flooding.

Cooranbong at Lake Macquarie had received 26.2mm of rain since 9.30am on Thursday at the time of publication, while Williamtown recorded 21.6mm.

Water remained on the road on the Newcastle Inner City Bypass near University Drive, as well as the Pacific Highway near Hank Street at Heatherbrae and the Pacific Highway near Richardson Road at Ferodale.

There is also water on the road at King Street near Medcalf Street at Warners Bay and on the Pacific Highway near Oakdale Road at Gateshead.

A two-car crash was reported on the New England Highway at Maitland just before 12.30pm.

Meanwhile, Hunter Water issued a statement on social media saying the heavy rain was putting pressure on the storm water system.

"We're closely monitoring the situation and will attend to overflows as soon as possible," the agency said.

"It's advised people avoid swimming in local beaches and waterways for two days after heavy rain events, like the one we're experiencing.

Heavy rainfall is putting pressure on our system, with widespread downpours across our area of operations. Heavy rain events lead to stormwater and debris infiltrating the wastewater system and can cause overflows."

Marine Rescue NSW is urging people to delay plans to go onto the water until the weather eases.

"Coastal bars will become extremely hazardous, with conditions offshore becoming increasingly dangerous with heavy rain and poor visibility, increasing seas and swell and strong winds," commissioner Stacey Tannos said.

"Even if you are intending to stay in enclosed waters, flooding on coastal rivers and estuaries may result in strong currents and submerged and floating debris.

"Navigation markers also may become submerged, moved or damaged and channels and shoals may shift. Boat ramps and moorings may be submerged, affected by debris or damaged.

"In these conditions, staying at home is the safest option. It's definitely not worth risking your life for a few fish."

Update - 10.30am:

This morning's drenching is showing no signs of letting up, with Williamtown recording 12mm of precipitation in the hour-and-a-half since 9am on Thursday.

According to Bureau of Meteorology figures, the Cooranbong gauge at Lake Macquarie had recorded 9.4mm in the same period, while Cessnock registered 6.8mm.

The BoM has also issued a marine wind warning for off the coast of the Hunter and some other parts of the state, which will remain active for the day.

There remains a significant amount of water on the road on the Newcastle Inner City Bypass near University Drive, as well as the Pacific Highway near Hank Street at Heatherbrae and the Pacific Highway near Richardson Road at Ferodale.

The State Emergency Service is reminding people to call 132 500 662 if they need help.

Earlier report:

A severe weather warning has been issued for the Hunter on Thursday with heavy rainfall predicted to hit the region across the next few days.

The State Emergency Service has warned the strong downpours could cause flash flooding in some areas after recent rain contributed to soaked catchments.

Williamtown recorded almost 44mm of rain in the 24 hours up to 9am Thursday, according to the Bureau of Meteorology. Falls between 50 to 100mm are forecast across the next few days with isolated heavy downfalls up to 200mm possible. Nelson Bay is expected to receive up to 120mm of rain on Friday alone.

Live Traffic NSW has reported traffic hazards on the Inner City Bypass at North Lambton and Kotara where there is water on the road.

"With many areas already saturated by a wet storm season, conditions are ripe for possible flash flooding," NSW SES Assistant Commissioner Sean Kearns said.

"We are making sure our volunteers are prepared and resourced to spring into action when needed.

"But just as our volunteers are prepared, we need our communities to be prepared as well.

"For weather events like this, that means knowing your flood risks. For instance, knowing the height of which your home, business and even your street could be affected by floodwater.

"It is also important to have an emergency kit ready to go with all your important documents and medications should you need to leave your home."

Mr Kearns also urged people to reconsider the need to travel.

"If you do need to be on the roads over the next few days, please drive to the conditions and check Live Traffic NSW for updates on road closures," Mr Kearns said.

"And if you come across floodwater while driving, never drive, walk, or ride through it - if the road is flooded, forget it.

"It doesn't matter how experienced a driver you think you are, or whether you have a big car, floodwater can wash away roads that once lay underneath, and even contain chemicals and debris."

The coastal trough is expected to deepen near the northern NSW coast during Thursday and then gradually extend south on Friday.

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