Cyclonic conditions along Australia's northeast coast generated huge seas washing away beaches at popular tourist spots and destructive winds and rain causing widespread flooding, authorities said.

Main Beach at Byron Bay, a popular tourist destination in northern New South Wales state, has all but disappeared, Byron Mayor Simon Richardson told media.

Television news footage showed a concrete walkway along the beach collapsing into the sea.

"Right now around Byron, we've got some severe weather, massive swells, we're watching our beach disappear," he said.

Byron Bay Cafe facing danger caused by erosion along the beach side

"What we've got here is yet another event. An extreme weather event coming on the back of climate change that our community's dealing with. It's about the fourth or fifth major event in the last couple of years."

The wet conditions contrast with the fierce bushfires that ravaged world heritage listed Fraser Island in Queensland state in recent weeks. Today, fire evacuation points on Fraser Island were underwater due to high tides and huge waves.

The heavy band of rain and wild winds, generated by an intense low-pressure system off the southern Queensland coast, battered the heavily-populated border regions between NSW and Queensland for the third day bringing more than 700 millimetres (27.6 inches) of rain in some places over 48 hours.

A floating waterpark breaks up during stormy conditions on Morton Bay at Redcliffe, Queensland

Australia's Bureau of Meteorology (BoM) warned coastal erosion, hazardous rain and huge waves off the coast would continue through today and urged motorists to stay off the roads.

Australia is expecting a wetter than usual summer this year due to a La Nina weather phenomenon, typically associated with greater rainfall and more tropical cyclones, though a major heatwave had sweltered the east just weeks ago.

"Major coastal erosion is ongoing along numerous beaches in northeast (NSW) and southeast Queensland as spring tides combined with large waves and gale force easterly winds eat away sand from beaches," BoM meteorologist Dean Narramore said.