Prepare to leave, NSW Premier tells thousands as flooding worsens

On Monday afternoon, Mr Perrottet stressed people in the state's 12 areas of warning should "begin preparing and planning for evacuation" where there is an evacuation warning in place."

The SES now has 44 current notifications across the state.

Hundreds of people across northern NSW have been left stranded for hours on rooftops amid the crisis as state and federal emergency services struggled to access impacted areas.

The Australian Defence Force has been called in help with rescues, as countless residents call out on social media and to SES for urgent rescues.

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Listen to Bridget McKenzie explain disaster arrangements

But wild conditions are making it difficult for four aircraft to join the effort.

The crisis is widening, with multiple major flood warnings for northeastern NSW including the Tweed, Richmond, Wilsons, Bellinger and Clarence rivers.

The extreme weather is forecast to move further south over the course of the week, with the premier flagging a potential future impact on the south coast, with emergency crews from SA and Victoria being brought in to help with the escalating crisis.

Minister for Emergency Services and Resilience Stephanie Cooke said the flooding that Lismore and surrounding areas were experiencing "is a natural disaster of unprecedented proportions".

Ms Cooke pleaded people across the state and in the Lismore region in particular "be on alert now more than ever. These conditions are different to anything you will have experienced before".

- with Australian Associated Press