The falls bring a sodden end to what is likely to be the city's wettest March in 42 years. The latest source of rain is the moisture being drawn in by the remnants of powerful ex-tropical Cyclone Debbie, which has combined with a cold front that has swept in from the south.

The Bureau of Meteorology has updated its severe weather warning covering coastal NSW from Sydney to the Queensland border. A separate warning is out for severe thunderstorms bringing potentially heavy rain to the lower Hunter including Newcastle and Gosford.

"24-hour totals in excess of 200 millimetres are expected over the Northern Rivers district during Thursday, and it is likely that some locations will exceed more than 350 millimetres," the bureau said.

For Sydney, the totals are likely to be 30-45 millimetres on Thursday. The mercury has now dropped towards the 20-degree mark as the front moves through, with "feel-like" temperatures recently about 15.8 degrees.

"There's a fairly long line of convected activity coming through ahead of that front and so we could see some thunderstorms pop up in Sydney," Helen Reid, a meteorologist with the bureau, said.

"Any time of day is a volatile one," Dr Reid said. "Once the southerly pushes through behind the cold front, it will stabilise and we'll be into a more showery-rain situation rather than [face] any thunderstorm potential as such."

For Sydney, any heavy rain is likely to be associated with a thunderstorm.

"Winds are going to be the problem," Dr Reid said.

Dangerous surf conditions are expected from the Illawarra to Queensland. On beaches in the state's north, waves exceeding five metres are forecast to develop, producing "significant beach erosion", the bureau said.

Weatherzone: Sydney radar

Tough commute home

The rain is likely to start to ease by the late afternoon, about the time the cold front arrives from the south, bringing strong winds with it.

"It's looking like the worse commute [of the day] will be on the way home [for Sydneysiders]," Tom Hough, a meteorologist with Weatherzone, said.

"Damaging winds averaging 65km/h with gusts in excess of 90km/h are possible along the coastal fringe from about Sydney to Forster during Thursday afternoon and evening, then extending to the remainder of the northern coastal fringe during Friday," the bureau said in its warning.

Up to Thursday morning, Sydney had had 282 millimetres of rain, the most in March since 1983. About 20 millimetres will get it past that year's total, making it wettest March since 1975, the bureau said.

(See chart below of the predicted rainfall totals in NSW on Thursday.)

The bureau has issued a flood watch for the Richmond-Wilsons Valley for moderate to major flooding, with several other regions such as the Clarence and Bellinger valleys potentially facing moderate flooding.

Temperatures to drop

Along with the wild weather, though, will come some welcome relief from the high humidity of recent days.

While overnight temperatures got down to 24.5 degrees in the city at 4.44am, the apparent - or "feels like" - reading was several degrees warmer throughout the night.

For instance, midnight in Sydney felt like 30.5 degrees.

Temperatures could drop below 20 degrees as the cold front moves through, Mr Hough said.

A few millimetres of rain are likely on Friday, with Saturday looking likely to be the driest of the days during the next week.

'Very dangerous'

Sydney's rain, though, is likely to be much easier to cope with than Brisbane's on Thursday, when 80-150 millimetres are expected.

Schools are closed from Brisbane to Agnes Water north of Bundaberg, the bureau also warning of a "very dangerous situation" as flood risks mount.

Ex-tropical Cyclone Debbie dumped 241 millimetres on Mackay on Wednesday, with hinterland regions topping a metre of rain in about 48 hours as the slow-moving storm battered the Queensland coast.

Tracking rainfall in Brisbane may be trickier than usual today, with the bureau reporting problems with its rain gauge.

The system covers most of the east coast, as this image below from the Himawari-8 satellite shows:

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