The death toll from flash flooding in Australia's northeast rose to seven on Sunday as a severe storm system pummelled the city of Brisbane and the southeast of the state of Queensland.

The state's premier, Annastacia Palaszczuk, compared the weather to a cyclone event, as Brisbane, the Queensland capital, was bracing for flooding on Sunday, before the rainfalls ease by Monday.

"Can I say to everyone from the outset today, if you don't have to be on the roads, please do not be on the roads," Palaszczuk said at a briefing on Sunday. "We have a lot of water rising right across Brisbane and the southeast at the moment."

A flood emergency has been in place across southeast Queensland, a state known for an abundance of sunshine. All Gold Coast and Sunshine Coast beaches, famous tourists attractions, have been closed.

More than 1,400 homes in Brisbane are expected to be impacted by possible flooding. Queensland emergency services said they have had 100 requests for assistance every hour in recent days.

About 700 people were asked to evacuate from the city of Gympie on Saturday after the Mary River system rose beyond a level of 22.06 m (72.4 ft), making it the worst flood in the town since the 1880s.

Six people have died in the Queensland flooding so far, including the late Saturday death of a 34-year-old man who tried to swim to safety when his car became submerged in floodwaters.

A man died in floodwaters in northern New South Wales (NSW) when his car was carried away, as the torrential rains move south, posing risks for New South Wales, Australia's most populous state.

"Significant thundery rainfall is expected over [northeast of NSW] though Sunday and into Monday. Life threatening flash flooding and major riverine flooding is possible," the NSW Bureau of Meteorology said on Twitter.