But the warm water is the result of a marine heatwave off the NSW coast, which a Newcastle scientist said is becoming more common and a threat to ecosystems under the waves.

Bill Leggat, associate professor at the University of Newcastle's school of environmental and life sciences, said the NSW heatwave is part of global trend.

"Recently, we have seen marine heatwaves becoming more and more prevalent," he said.

"I've been working in this field for over 20 years and it wasn't until the last decade that we even needed to start talking about these kinds of events."

The temperature at Newcastle beaches was up around 24 degrees on Thursday.

Satellite images show a warm water band affecting much of the central NSW coast, with Sydney experiencing water temperatures three degrees above average in the past few weeks.

A marine heatwave occurs when a location sees ocean temperatures in the top 10 per cent ever recorded during that time of year, for at least five straight days.

This has to do with a number of "complex" factors such as weather patterns and water currents but Professor Leggat said "climate change" was a key driver.

The Marine Heatwaves International Working Group tracks heatwaves and their severity across the world.

The organisation currently has the NSW heatwave listed as "strong", a category they say is on track to overtake the less severe, "moderate", as the most common heatwave category.

"This category is known to have limited effect on marine organisms but if the heatwave becomes any more intense it will have a widespread impact," Professor Leggat said.

The National Oceanic and Atmospheric Administration predicts the marine heatwave to increase in intensity over the next three months.

While corals are the "canary in the coalmine", according to Professor Leggat, marine life across Newcastle's coastal areas may be impacted by the increase in water temperatures.

"This can be on kelp, seagrass and oyster populations. These fundamental organism can't escape to cooler water and if they are impacted it will have an effect through the whole ecosystem."

In 2015, a marine heatwave caused "irreversible" damage to Tasmania's kelp forests.

"We compare these events to terrestrial warming and bushfires," Professor Leggat said.

"People have this idea that a slow global temperature increase isn't too bad. But it has consequences like bushfires, floods, cyclones and marine heatwaves."

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