NSW has been hit by successive major floods in recent months, with the Northern Rivers area devastated by two deluges within weeks and Sydney enduring an almost constant drenching.

Bureau of Meteorology Senior Hydrologist Ailsa Schofield confirmed on Thursday that Sydney's Observatory Hill site had already copped more than the mean annual rainfall despite it only being early April.

Ms Schofield said the yearly total is likely to continue rapidly climbing until the La Nina weather phenomenon passes.

"Unfortunately we continue to be in a La Nina event, which we know for NSW means that we can expect to see higher-than-average rainfall conditions - which is exactly what we have seen over the past couple of months," she said

"We are expecting La Nina to continue throughout the remainder of April."

Last month Sydney was drenched with its wettest March on record, with 537mm falling into the Observatory Hill gauge.

This broke the previous high for the month of March of 521mm which fell in 1942.

The record made it a "very significant month", the BOM's senior meteorologist Jonathan How told AAP.

"What we do know is that it's the second-fastest on record that Sydney has surpassed 1000 millimetres for the year," he said.

More than 114mm fell at Observatory Hill in the 24 hours up to 9am on Thursday, and a further 18.4mm in the four hours that followed, taking its total for the year to 1245.2mm.

The annual average is 1213.4mm.

Heavy falls have battered the Central Coast, Greater Sydney, Illawarra and South Coast, and the rainfall is forecast to continue through Friday and into the weekend, Ms Schofield said.

Cronulla, south of Sydney's centre, recorded 107mm in three hours overnight.

Meanwhile in the Illawarra, Mount Pleasant received 112mm and Rixons Pass 79mm in less than four hours on Thursday.

Australian Associated Press