From fires to floods: Severe rain and high winds challenge the state of NSW

The state of New South Wales is being drenched in some of its heaviest rain in 20 years. 

Residents have been warned to take the flooding seriously as severe rain and high winds bring a new challenge to NSW.

Australia correspondent Jason Morrison says it's yet another crisis for Australia.

"It's gone from being the fire situation now to the flood situation," he told The AM Show on Monday. "It's pretty ferocious."

Thousands of Sydney residents had to be evacuated on Sunday night after the downpour. It prompted dozens of rescues including motorists trapped by fallen trees.

Rapids formed on the Parramatta River which overflowed onto walkways. The rain is not letting up - with fears the worst is yet to come.

Morrison said Monday will be a tough day for the state. 

Australia's Bureau of Meteorology said on Sunday night the heavy rain had caused major flooding in several areas of NSW and conditions would be closely monitored overnight.

While these conditions could be destructive, they do provide some much-needed relief to bushfire-ravaged areas.

Morrison told The AM Show that time will tell if the rain hit every spot it needed to.

"They call it drought-breaking; I wouldn't quite go there yet - but hey what a great turnaround from where we were two or three weeks ago."