Flash flooding on Australia's southeast coast has killed three people and cut off towns, isolating tens of thousands of residents, as officials on Thursday warned more downpours were expected over the next 24 hours.

Four people have died and several were missing as major flooding in New South Wales (NSW) left about 50,000 people isolated by floodwaters. 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.