The two systems have sat over the New South Wales, Victorian and South Australian border and northern NSW since late last week but will now move to combine over the southeast coast on Tuesday.

The two systems have sat over the New South Wales, Victorian and South Australian border and northern NSW since late last week but will now move to combine over the southeast coast on Tuesday.

Weatherzone meteorologist Joel Pippard explained low-pressure systems either have a cold core, which causes heavy rain during the day and cold fronts, or a warm core, which causes more widespread heavy rain that peaks during the night. 

Current radar images show the system sitting over northern NSW is a warm core system while the one over the Victorian, NSW and SA border has a cold core.

'As the name suggests, a hybrid cyclone refers to a low pressure system that has both warm and cold air in its core. This mix of air leads to a mix of energy sources, which can allow hybrid cyclones to intensify quite rapidly,' Mr Pippard said.