The Warragamba Dam began spilling over at 3am on Wednesday morning, prompting safety warnings for the surrounding areas. SES crews are now doorknocking the affected zone as the evacuation begins.

"Please do not contemplate staying," said NSW Deputy Premier Paul Toole.

A body has been recovered from Lismore, NSW bringing the known death toll to four.

"This is a catastrophic event, we know that we may see more lives lost over the coming days."