Australia flooding: 'Major' deluge continues in New South Wales, increased rain expected on Monday

Intense rainfall is continuing in Australia's New South Wales with forecasters predicting Monday could be the worst day yet.

Authorities have been pleading with people to stay at home in Australia's biggest city of Sydney, with flooding likely to be the worst the state has seen since 1961.

"Rain has continued across NSW and the ACT overnight, with the heaviest falls across the Mid North Coast and Northern Rivers districts," NSW's Bureau of Meteorology said in a statement early on Monday. 

Officials have issued an order for many schools to stay shut on Monday due to the severe weather.

Footage shows increased damage across NSW, with people being winched into helicopters from rooftops as their homes became swollen by floodwaters.

On Sunday evening, about 300 rescues had taken place in the previous 48 hours, Newshub Australia correspondent Emma Cropper said.

The Hawkesbury River began overflowing on Sunday afternoon at North Richmond, a semi-rural suburb in NSW.

"Flooding is likely to be higher than any floods since November 1961," NSW's State Emergency Services said.

People continue to be urged to stay at home and avoid non-essential travel.

"We are now envisaging a one in 50-year event, yesterday we were hoping it would only be a one in 20-year event," Berejiklian told a news briefing on Sunday. "That means potentially another 4000 people may be asked to evacuate."