Up to 300mm of rain has fallen in the NSW coastal resort town of Byron Bay overnight as millions of people brace for flooding road closures and landslides as heavy rain continues on Friday.

The New South Wales Bureau of Meteorology said 281mm of rain had fallen in Bryon Bay alone as of 8am on Friday while the SES responded to 340 calls for assistance across the state.

The Byron Bay News reported shops and homes were inundated and the deluge is reportedly the highest rainfall total in decades to hit the northern NSW coast.

The bureau issued a severe weather warning on Friday morning for the coast from the south of Wollongong up to Lismore in NSW's north.

Sydney, Wollongong and Newcastle could all be affected by further flooding, the bureau said.

One local resident posted on social media that going shopping in Byron Bay "now requires a wetsuit".

The weather bureau's warning came after downpours on Thursday night brought heavy rain and flooding to much of the state. The heaviest falls have been on the NSW north coast, with several areas recording more than 200 millimetres by Friday morning.

In south east Queensland, flashing flooding has caused road closures after Tin Can Bay and Coops Corner recorded 395mm in the last three days while Eumundi has had 340mm.

The Bureau of Meteorology's Jonty Hall said the Eumundi and Yandina area on the Sunshine Coast recorded 90 millimetres of rain in an hour overnight.

"It is a part of the world which is no stranger to heavy rainfall but even for there that was pretty heavy," he told the ABC.

'Border to border': Flood warnings for NSW, Qld amid massive downpours_1
Heavy rain is setting in across a broad area of eastern NSW, prompting a mix of joy and concern. Photo: NSW Bureau of Meteorology

"What we're going to expect today is that rainfall to shift further south," BOM forecaster Gabrielle Woodhouse said.

"We'll start to see the focus of that rain through southern parts of the mid-north coast, Hunter and then into Sydney and Illawarra going into tonight."

Penrith is forecast to receive up to 100 millimetres on Friday and up to 150 millimetres on Sunday. Sydney can expect up to 90 millimetres on Friday and Saturday.

Significant falls were also predicted over Sydney's main drinking water catchments, which are at just 42 per cent capacity.

The NSW State Emergency Service said it had received nearly 340 calls for assistance across the state since the rain started.

Most were related to water over roads, roof damage and downed trees.

In the state's northern rivers region, a car was washed off the road and two people had to be rescued from a caravan due to rising floodwaters.

"It hasn't been anything too concerning so far but we've had to do a couple of rescues due to isolation and there have been a lot of reports of roof damage and water coming into properties," SES spokesman David Webber said.

Assistant Commissioner Paul Bailey told the ABC that the SES had "rescue crews right up and down the coast because we know this event will really go border to border".

'Border to border': Flood warnings for NSW, Qld amid massive downpours_2
Heavy rain is forecast for the area shaded in yellow. Image: BOM

The road to the Jenolan Caves, south-west of Katoomba, has already been closed due to fears the heavy rain could cause landslides.

"There are also a significant number of loose rock fragments, which have an increased likelihood of rolling down the slope and on to the road," a Transport for NSW spokesman said.

"Given current weather forecasts are predicting heavy and prolonged rainfall in the area over the coming days – the first in the area since the bushfires – it is essential for safety reasons that the road is closed until it can be assessed safe for traffic."

Several roads north of Sydney were closed due to flooding on Friday morning, and trains on the T1 North Shore line were delayed due to a fallen tree branch.

Buses also replaced ferries between Parramatta and Rydalmere because the Parramatta Weir was overflowing.

NSW Maritime executive director Mark Hutchings said conditions were also dangerous out at sea.

"Skippers and owners of small boats should be on high alert and check the conditions before considering crossing a coastal bar or heading offshore," he said.

The NSW Rural Fire Service said the heavy rain was welcome in bushfire-ravaged parts of the state.

"We were over the moon to see rain arrive across many parts of NSW, with decent falls in the state's north," the RFS said on Thursday night.

-with agencies