Huge tides, gale force winds and heavy rain have caused "massive" coastal erosion on Byron Bay's Main Beach. A walkway has collapsed into the ocean and there are now concerns for beachfront properties.
Pictures show the sand on Main Beach has been inundated with water overnight and is clogged with fallen trees.
Bureau of Meteorology senior meteorologist Dean Narramore says torrential rain will continue from Brisbane down to Yamba in northern NSW, with 100 to 250mm expected in the next 24 hours.
"This morning we've observed waves offshore around the eight metre mark," he said.
"We'll see a big king tide today, very high tides again tomorrow.
"We could see waves of three-to-five metres come into shore ... today and into tomorrow."
There were more than 50 calls for help overnight to the NSW State Emergency Services as wild weather pummelled the mid north coast, with Tweed Heads and Mullumbimby the worst areas affected.
There is no reprieve in sight with the BOM warning northern NSW will continue to be hit with heavy rain, damaging winds and flooding, as well as dangerous surf conditions until the weekend.
A flood watch and a severe weather warning are current for the northern rivers and mid north coast.
The BOM has issued a moderate to major flood warning for the Bellinger River at Thora and Bellingen which will affect the towns of Byron Bay, Tweed Heads, Lismore, Grafton, Coffs Harbour, Bellingen, Nambucca Heads and Kempsey.
It is warning about more showers and storms next week, with those conditions continuing right until Christmas.
The low-pressure system off the southeast Queensland coast is expected to bring another round of very heavy rainfall and strong and damaging winds, with gusts of more than 100 km/h in some areas forecast.
Queensland Fire and Emergency Services Commissioner Greg Leach said the worst affected areas were from Ipswich through to Logan and the Gold Coast and northern NSW.
The Gold Coast and parts of its hinterland copped 193mm of rain overnight on top of the 460mm they received on Sunday.
An emergency alert was issued on Sunday night on the Gold Coast for the areas of Tallebudgera and Currumbin, and the SES responded to about 690 calls in the last 36 hours.
The NSW SES said more than 700 calls for help had been made since Friday and 57 overnight, while a severe weather warning for very heavy rainfall is current for the Northern Rivers areas.
NSW SES Deputy Commissioner Daniel Austin said the BOM had issued numerous flood watch messages for a range of catchments right across the state's north.
"So anybody living up in that area needs to be aware of what is going on around them. Listen to the warnings. And as always we ask that people don't drive through flood waters," he told the Today show.
The SES said heavy rainfall over Leycester Creek and the Wilsons River at Lismore is likely to reach the minor flood level (4.20 metres) around midday on Monday, with roads and low lying farm land and rural properties most likely to be inundated with flood waters.
Further rises to the minor flood levels are possible at Billinudgel and Mullumbimby. Minor flooding is expected along Marshalls Creek and the Brunswick River.
The Tweed River at Tumbulgum is likely to reach the minor flood level (1.40m) on Monday with the high tide.
The SES says the flood situation is being monitored and revised warnings will be issued should the situation change.
People in areas likely to be impacted by flooding should refrain from driving or walking through flood water, take note of road closures and monitor emergency warnings and severe weather updates.
Main image: Scarborough in the Moreton Bay region. John Hammond. Supplied.