“Intense rainfall which may lead to dangerous and life-threatening flash flooding may develop in areas of the Mid North Coast and northern Hunter districts as continued rainfall impacts the region,” it said. “Roads are likely to be cut off by water, and there will be an increased risk of landslips over this period.”

The bureau also issued a road weather alert for Sydney drivers, warning of “dangerous” conditions in all suburbs on Thursday. “Motorists are advised to take extreme care,” it said.

Ben Domensino, a senior forecaster at Weatherzone, said Sydney was likely to receive 100-200 millimetres between Thursday and the weekend, with the heaviest falls likely on Friday into Saturday.

The Bureau said Sydney’s falls were likely to range from 35-50 millimetres on Friday and 60-120 millimetres on Saturday, with storms possible both days. Each day out to next Thursday will also likely have showers.

“It’s a blocking high in the southern Tasman Sea that is causing huge amounts of moisture to be feeding into eastern NSW,” Mr Domensino said. “That’s combining with cool upper level area that is stirring instability and bringing rains and storms.”

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While winds will pick up as the developing low moves southwards on Friday, beaches are not likely to cop significant erosion in coming days because the system is too close to the coast to develop a big swell, he said.

Still, the bureau has issued a warning for hazardous surf for the Coffs, Macquarie and Hunter coasts for Friday.

A second pool of cool air will move eastwards from early next week, bringing the potential for heavy falls in western and central NSW, as well as more coastal showers, Mr Domensino said.

That rain, coming on top of falls of 60-80 millimetres in recent days for places such as Dubbo and Walgett, will swell rivers and bring some flood risks for inland areas, he said.

The Bureau has issued a flood watch for moderate to major flooding for the Bellinger, Kalang, Paterson and Williams rivers.

An initial moderate flood warning was also issued on Thursday night for the Orara River, expected to exceed the minor flood level of four metres around 11.10pm at Glenreagh Bridge and five metres around midnight at Couts Crossing.

The river level may reach eight metres at Glenreagh Bridge on Friday morning and 9.3 metres at Coutts Crossing on Friday afternoon, bringing moderate flooding.

The burst of autumn rain comes as the La Nina weather pattern in the Pacific “is on its last legs”, Mr Domensino said. Sea-surface temperatures are back in a neutral range but the atmosphere is still in a mode favouring wetter-than-average conditions for eastern NSW into April.

The soaking rains mean that the fire season will officially come to an end on March 31.

“It’s the first time for a long time, there’s been no extensions [of fire restrictions] anywhere in NSW,” Ben Shepherd, a senior RFS spokesman said. “It’s the quietest fire season since 2010-11.”

Figures compiled by the RFS show that, between July 1 and the end of February, the area burnt in the state was 57,138 hectares, or just 1 per cent of the 5,490,504 hectares during the comparable period in the previous year. There were 5132 fires or fewer than half the tally of 11,482 a year earlier.

“In the 2020-21 fire season to date there have been no Class 3, or Section 44 bushfires” that trigger a so-called bushfire emergency, Mr Shepherd said. That’s also the first time a decade without one.