"Once-in-a-century" floods have turned streets into rivers and forced thousands to abandon their homes in northeast Australia, with authorities warning of tornadoes and more rain over the next few days.

Australia's tropical north experiences heavy rains during the monsoon season at this time of the year, but the recent downpour has surged far above normal levels.

Thousands of residents in the city of Townsville in Queensland were without power and cut off by flooded roads.
More severe weather could whip up tornadoes and destructive winds in the days ahead, Bureau of Meteorology state manager Bruce Gunn said on Sunday.
Up to 20,000 homes are also at risk of being inundated if the rains continue.

Military personnel were delivering tens of thousands of sandbags to affected locals, as Queensland premier Annastacia Palaszczuk warned residents to be careful.

"It's basically not just a one in 20-year event, it's a one in 100-year event," she said on Saturday.

The Bureau of Meteorology said a slow-moving monsoonal trough was sitting above Queensland, with some areas expected to receive more than a year's worth of rain before conditions ease.

Bureau meteorologist Adam Blazak said the heavy downpours could continue until Thursday, while floodwaters would take some time to recede even when the rains lessen.

The region receives an average of some 2,000 millimetres of rain annually but some towns were already on track to pass that.

The town of Ingham, north of Townsville, received 506 mm of rain in 24 hours between Saturday and Sunday, of which 145 mm fell in just one hour, Blazak said.

"I've never seen anything like this," Townsville resident Chris Brookehouse told national broadcaster ABC, adding that his house was flooded with water more than one metre high.

"The volume of water is just incredible. Downstairs is gone, the fridge and freezer are floating. Another five or six steps and upstairs is gone too."