Key Points

The State Emergency Service (SES) in NSW late on Monday night issued emergency warnings for residents of the towns of Dungog and Paterson, over 140 km north of Sydney in the state's Hunter region, advising them to evacuate by 11:30 p.m.

"If you remain in the area, you may become trapped without power, water, and other essential services. It may be too dangerous for NSW SES to rescue you, and buildings may not be able to withstand the impact of flood water," it said.

Separate warnings issued in the early hours of Tuesday morning for parts of the Hunter towns of Bulahdelah and Gloucester urged residents to evacuate before 6 a.m.

Regions north of Sydney have been hit by heavy rainfall since Sunday, which has caused widespread flash flooding.

SES Assistant Commissioner Colin Malone told the Australian Broadcasting Corporation on Tuesday morning that the evacuation orders were issued late at night after river levels hit trigger points earlier than forecast.

The SES responded to over 2,000 calls for assistance across NSW in the 24 hours to Tuesday morning, 1,400 of which prompted emergency responses.

SES state operations Chief Superintendent Dallas Byrnes said that 22 of the emergency response incidents were flood rescues, the majority of which were people who were taken by surprise by flooding, Xinhua news agency reported.

Parts of the Hunter and the neighbouring Mid North Coast region received over 200 mm of rainfall over 24 hours.

Flights in and out of the airport in Newcastle, the largest city in the Hunter, have been cancelled or disrupted, and more than 30 schools have been closed.

A severe weather warning issued by the Bureau of Meteorology shortly after 3:30 a.m. on Tuesday said that intense rainfall and damaging winds may continue into Wednesday and cause "dangerous and life-threatening" flash flooding.

As of Tuesday morning, about 6,000 properties in the Hunter, Mid North Coast and Central Coast were without power.

- IANS