Pedestrians take cover from the rain on Thursday. Photo: Daniel Munoz

The state's second east coast low in as many weeks is likely to bring more  heavy rain to north-eastern NSW but largely skirt flood-hit areas around Sydney and the Hunter Valley, meteorologists say.

Sydney  could  expect about 15mm of rain on Friday with falls of about 20-30mm on Saturday and Sunday before skies start to clear, Mohammed Nabi, a duty forecaster with the bureau, said.

While there was  a chance for thunderstorms bringing heavier localised falls to the city for Friday to Sunday, the bursts were likely to be short and sharp rather than long-lasting, he said.

"We are starting off with cloudy days ... so we won't have the right temps to generate big thunderstorms," he said.

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The wet start to May follows Sydney's wettest April in 26 years with about 367mm – or close to triple the average – filling the city's rain gauges.

The bureau was  predicting mild overnight temperatures for Sydney of 17-18 degrees for the coming five days, with maximums gradually warming to 24 degrees by Tuesday.

The east coast low, also known as an east coast cyclone, was predicted to form during Friday near the south-east Queensland coast and gradually move southward. The system was  already bringing heavy falls, such as 75mm in an hour on Thursday morning to the Wooli River area near Grafton. 

Thunderstorms were also forecast for late on Thursday for a coastal strip from just north of Newcastle all the way to Queensland, with the bureau warning of possible flash flooding in areas from Taree to Coffs Harbour and Lismore. An earlier flood watch from the bureau also identified five rivers in the region that may cop major flooding during coming days.

Over three days, parts of the state's north-east may get 350mm of rain in total. Friday and Saturday would likely bring widespread falls of 50-100mm with some localised falls of 200mm or more, Mr Nabi said.

By Sunday, the largest falls were  likely to be over the mid-north coast, with some areas likely to receive more than 100mm, as shown in the following four-day rainfall forecast:

Just as Sydney would  likely dodge the heaviest falls and strongest winds, areas in the Hunter Valley should also avoid sustained deluges  – good news for construction crews still making repairs after last week's storms.

"It looks like the system spares the Hunter quite a bit," Mr Nabi said, adding that the three-day rainfall totals in the valley to Sunday would likely be "100mm at the most".