Meanwhile, the Bureau of Meteorology has put the Top End and Kimberley on Tropical Cyclone watch, with a likelihood that Cyclone Vernon might form.

"If you're in Darwin, the Top End or parts of the Kimberley, now is the time to prepare," said the Bureau of Meteorology's Jonathon How.

More than 100mm fell over the Mid-North Coast and Central Coast of NSW on Thursday.

Wamberal Reservoir on the Central Coast received up to 140mm on Thursday, while Belligen on the Mid-North Coast recorded 173mm.

The rain is expected to move toward the Queensland border, bringing heavy downpours to the state's south on Friday.

The Bureau of Meteorology's Dean Narramore told media on Thursday that an offshore surface trough will bring the high rainfall.

Some areas around Byron Bay and K'Gari Island could see in excess of 300mm in the next few days.

"This is a dangerous set up as very heavy rainfall falls over already saturated soils," Mr Narramore said on Thursday afternoon.

Low lying areas around Nambucca River have been evacuated due to flood risks.

The SES recorded 630 callouts in the 24 hours to 6am on Thursday, including 27 floodrescues. The majority of which have been from vehicles.

The Northern Rivers, Mid-North Coast and Central Coast are the areas of concern for the SES on Friday.

SES Assistant Commissioner Dean Storey told the ABC on Thursday there is a "very concerning weather system ... sitting just off the coast and due to make landfall".

WeatherZone has reported that Sydney has recorded its wettest summer in 30 years with its seasonal rainfall hitting nearly 520mm after being lashed with more than 120mm in 48 hours.

While Tasmania is on track to have one of its three driest summers on record.

The BoM has also advised that autumn will likely see high rainfall targets as well.

- with Australian Associated Press