Torrential rain is lashing Australia's southeast coast, forcing thousands of people from their homes because of the danger of floods and landslides in a region battered by major flooding four times in the past 18 months.

Some areas in New South Wales state have received a month's rain in the past two days, swelling rivers and forcing Sydney's main dam, the Warragamba Dam, to spill on the weekend.

Flooding in March and April in New South Wales and southeast Queensland state resulted in 4.8 billion Australian dollars ($3.3 billion) in insured damage, the Insurance Council of Australia estimated.