People across the eastern seaboard have been warned not to underestimate an already-fatal deluge of rain expected to bring more flooding to two states.

An inland low and coastal trough joining forces over New South Wales is forecast to dump up to 300mm of rain on Sydney, Newcastle and coastal towns on Friday.

The widespread heavy falls follow intense rain on either side of the Queensland-NSW border on Wednesday night and Thursday morning.

The storms flooded rivers and inundated roads resulting in one man drowning when his car was swept away and others needing to be rescued in south-east Queensland.

Tweet from @BOM_au

Animals also had to be rushed to higher ground as water engulfed Byron Bay Wildlife Sanctuary on Thursday.

“The forecasts are quite severe,” NSW SES Metro Zone Commander Allison Flaxman said.

“We don’t want anybody under-estimating the impacts this could have on you.

“Prepare now while you have the chance, stay off the roads if possible and definitely don’t drive through floodwaters.”

Tweet from @BOM_au

The message came as Queensland police investigate the death of a 71-year-old man whose ute was found submerged in a flooded creek near Logan, south-west of Brisbane.

Acting Police Commissioner Steve Gollschewski said it was early days in the inquiry but the incident was “clearly an absolutely tragic circumstance”.

Several other people in the region also drove into floodwaters overnight and needed rescuing, Gollschewski said.

Three more drivers were saved from serious injury in Murwillumbah as a sudden and intense rainstorm swamped the NSW north coast on Thursday.

The storm also took out perimeter fences at Byron Bay Wildlife Sanctuary, damaged enclosures and buildings and forced the relocation of the animal hospital to higher ground.

‘A taste of what’s to come’

“The threat of more rain hangs over us,” the sanctuary said in a social media.

“As a result, the sanctuary will remain closed until further notice as we work tirelessly to assess risks and restore our haven for wildlife.”

Meteorologist Angus Hines said the significant surface flooding across the region was “just a taste of what’s to come over the next three days” for the eastern seaboard.

“It is very wet,” Hines said.

“Flash flooding and riverine flooding are both possible and this is going to be a dynamic and serious situation.”

Major flooding was predicted along the Bremer River on Thursday while moderate riverine flooding was expected in much of southern inland and south-west Queensland.

For residents near the Moonie and Condamine rivers it marks just a few months since river levels rose and flooded homes in January

Nervous wait

A nervous wait is in store for residents along the Hawkesbury and Neapan rivers that bound Sydney, one of several areas warned to expect 50 to 90 mm within six hours on Friday.

Localised 24-hour totals of 120mm to 200mm are possible in southern Sydney and along the NSW south coast, intensifying over the Illawarra escarpment where up to 300mm is feasible.

Damaging winds are forecast, with gusts up to 90km/h possible along the coast south of Sydney.

A flood watch is in place for the Mid North Coast, Wollombi Brook, Sydney region, South Coast and parts of the north west.

The Hawkesbury Nepean River could rise to major flood levels from late Friday.

Conditions are predicted to ease by Saturday afternoon.

-AAP