Hundreds of New South Wales residents have made calls for help as heavy rain lashes the state, leaving huge swathes at risk of flooding, with authorities warning of more heavy rain to come.

Sydney, which has been drenched by torrential rains and was hit by more wet weather on Thursday, recorded its wettest August for “quite some time”, according to the Bureau of Meteorology.

The NSW State Emergency Service (SES) had more than 30 warnings, including several “watch and act” alerts, in place for areas stretching from south of Sydney up to the Central Coast, the north coast, the New England region and the state’s north-west.

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Debbie Platz, the deputy commissioner of NSW SES, told the ABC the weather system was “very widespread” and had caused “significant” rain both on the NSW east coast and inland. The SES has responded to 430 incidents over the past 24 hours.

The SES has warned of the risk of flash flooding with already elevated river levels, saturated soils and full dams from other recent significant rainfall.

The Bureau of Meteorology’s Helen Reid said the mid-north coast could receive isolated rainfall of up to 150mm on Thursday, with totals as high as 100mm in the northern rivers and 70mm in coastal Sydney.

“[There are] plenty of rivers on the rise, so just keep an eye on them,” she said.

Sydney Observatory Hill recorded 82mm in the 24 hours up to Thursday morning, while eastern parts of the city recorded between 60-70mm, and western Sydney 30-40mm.

Sydney, which has been drenched by torrential rains and was hit by more wet weather on Thursday, recorded its wettest August for ‘quite some time’, according to the Bureau of Meteorology. Photograph: Steve Markham/AAP

Reid said persistent rainfall was forecast to peak on Thursday and would ease into the evening and Friday morning, with totals closer to 5-10mm in parts of coastal NSW.

Reid said although the month was not done, it appeared to be “the wettest one for quite some time”.

Reid said a coastal trough was “teaming up with a [section] of the upper atmosphere that is a little bit cooler and unstable, and that’s really over central northern New South Wales at the moment”.

“That combination is helping a lot of moisture that’s fed in from tropical regions give us consistent rainfall across a broad area.”

A “watch and act” warning was in place for Goangra in the state’s north, where heavy rain was forecast to bring the risk of renewed moderate to major flooding.

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The BoM issued a major flood warning for the Namoi River, while minor flood warnings were in place across the Peel and Nepean rivers and the Barwon River on the Queensland border.

NSW police said divers would assist with the search for two men who were missing after a car entered a river 20km north of Wisemans Ferry, on Sydney’s northern outskirts.

Police were called to the Macdonald River at St Albans at about 11.50pm on Wednesday, where they found the car had crashed into the river.

One passenger, a man in his 20s, was able to exit the vehicle, but two men were unaccounted for.

Inspector Claudette Gebrael from Hawkesbury Police Area Command said police believed the trio were a father and two sons.

Assistant commissioner Colin Malone said residents should know the risk to their properties and start preparing their homes by clearing gutters and downpipes, trimming trees and tying down loose items.

“We know the weather and flooding this year has been tiring for many of you, but please, stay ready and stay safe,” he said, adding people should never drive through flood waters.