Areas of Australia that have seen their worst flooding in decades are bracing for more heavy rainfall, with the peak expected on Tuesday.

The entire coast of New South Wales has been put on alert, with troops and hundreds of volunteers deployed.

Roads and bridges have been cut off, cars and signs submerged, livestock marooned and schools closed.

PM Scott Morrison told MPs there was "serious risk still ahead" and weather officials said "it's far from over".

New South Wales (NSW) Premier Gladys Berejiklian said no deaths had yet been reported - "a miracle given what we have been through"

Some 18,000 people have been evacuated so far in the state, which is Australia's most populated with eight million residents.

What is the latest forecast?

The heaviest rainfall for the worst-affected state of NSW is expected overnight into Tuesday morning, as two major weather systems collide. The southern coastal areas could be the worst hit this time.

A tweet put out by Australia's Bureau of Meteorology said an area as large as Alaska - stretching from the NSW coast back to the southern Northern Territory - was now being affected by weather warnings.

In all, 10 million people are under warnings in every state and territory except Western Australia.

A low pressure weather system that has been inundating the NSW coast for days has now been met by another weather system inland that is moving east.

The bureau has forecast "increased rainfall, strong winds, damaging surf and abnormally high tides" in New South Wales on Tuesday.

"It may have been going for days but unfortunately this situation is far from over," the bureau tweeted.

Its flood manager, Justin Field, said: "I've been a flood forecaster in the bureau for 20 years and this is probably the worst flooding that I've experienced and I've had to forecast.

"We've got a flood watch that covers all the way from the Queensland border down to the Victorian border."

PM Morrison told MPs on Monday: "Across NSW, 1,400 first responders have conducted over 700 flood rescues and responded to over 7,500 requests for assistance."

He added: "This is an ongoing situation that is evolving and is extremely dangerous."

The military is being deployed to help with search and rescue, in what has been called a "one-in-50-years event".

Some areas have seen close to a metre in rainfall.

But the bureau did predict some respite, tweeting: "Most areas of New South Wales will see a clearing trend early Wednesday as a drier airmass moves into the region."

What has been the damage so far?

Days of torrential downpours have caused rivers and dams to overflow around Sydney - the state capital - and in south-east Queensland.

Some 200 schools in NSW will remain shut on Tuesday as the peak rainfall hits.