Sites in NSW's Northern Rivers District had about 400mm of rain in just a few days, the Bureau of Meteorology's Jane Golding told reporters on Monday.

Some locations near the Queensland-NSW border were inundated with 180mm of rain on Sunday.

And there was no relief as conditions deteriorated throughout Monday.

Wind gusts have been as high as 104 km/h at Cape Byron and are increasing, with the weather bureau warning of winds strong enough to bring down trees and power lines.

Other beaches along the NSW northern coast are being eroded after huge tides, gale force winds and heavy rain washed away much of the Main Beach at the popular tourist town of Byron Bay.

"We're seeing the largest coastal erosion we've seen in many years, particularly around the Byron Bay area, which is completely changing the entire landscape of the beaches," Surf Life Saving NSW CEO Steve Pearce told reporters.

Powerful waves are expected to continue to batter the NSW mid-north coast, almost as far south as Newcastle.

Authorities have asked residents to be prepared for dangerous conditions, warning them to protect their homes and animals, stay on top of the latest warnings, and avoid driving through flood waters or swimming at beaches.

The rains will likely ease later on Tuesday.

Federal Emergency Management Minister Stuart Littleproud said the government was "very concerned" about the weather system, but agencies were prepared.

"We knew that we would have to pivot quickly from bushfires to La Nina.... We have been ready for this, we have practised for this," he told ABC TV.

The NSW State Emergency Service had received more than 900 requests for help by Monday afternoon, with 160 logged between 6pm on Sunday and 1pm on Monday.

Four people have had to be rescued, including a woman who was swept away when she tried to leave her car after getting caught in floodwaters.

The rain has produced significant overland flash flooding with more flooding likely to continue even after the worst of the rain subsides.

The intense weather could create permanent rips along beaches that were previously safe, Surf Life Saving NSW says.

In southeast Queensland, the threat of damaging winds and heavy rainfall had eased by Monday evening but surf and tide hazards persist, the BOM warned.

The conditions in southeast Queensland had previously been comparable to a tropical cylone, Ms Golding said.

Sunshine and Gold Coast beaches were closed on Monday and coastal areas suffered major erosion.

The BOM has issued a severe thunderstorm warning for heavy rain in the Wide Bay region north of Brisbane, including Bundaberg.

It is warning about more showers and storms next week, with those conditions continuing right up to Christmas.

A flood watch is current from Fraser Island to the NSW border and there is a minor flood warning for the Logan River.

The low-pressure system off the southeast Queensland coast is expected to bring another round of very heavy rain and strong and damaging winds, with gusts of more than 100 km/h in some areas forecast.

The weather bureau also warned of severe thunderstorms in Darwin's Top End on Monday, with possible flash flooding in places including Borroloola, Minyerri and the Carpentaria District south of Ngukurr.

Australian Associated Press