The record rainfall, now declared a natural disaster, has been caused by a slow-moving area of low pressure and is greater than any in living memory for some residents, according to local authorities.

Heavy rains moved south overnight affecting Sydney and Newcastle, with the Bureau of Meteorology issuing warnings for southern parts of the state, BBC reported.

"Tragically, we're seeing more extreme weather events," said Australia's Prime Minister Anthony Albanese during a visit to flood-hit areas, adding "they're occurring more frequently and they're more intense".

"Our thoughts are with communities that are cut off...we're here to basically say - very clearly and explicitly - you are not alone."

Albanese thanked the emergency services for responding to hundreds of rescue calls, saying "at the worse of times, we see the best of the Australian character".

The NSW State Emergency Service (SES) said it had responded to more than 535 flood rescues in the 24 hours to 05:00 local time (19:00 GMT) - and more than 670 rescues since the flooding began.

There are more than 150 flood warnings in place with 40 of those at emergency level and authorities have re-issued earlier advice for people not to drive or enter floodwaters. (NMA)

•PHOTO:TEHRAN, May 23 (MNA) – Four people have died and several were missing as major flooding in New South Wales (NSW) left about 50,000 people isolated by floodwaters.