Up to 20,000 homes are at risk of being inundated if the rains continue.

Thousands of people in northeast Australia should expect "unprecedented flooding", authorities have warned, after relentless downpours forced a dam to be fully opened on Sunday.

Once-in-a-century floods have turned streets into rivers and caused thousands to abandon their homes in Townsville, in Queensland state.

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.

The Bureau of Meteorology late Sunday issued a "major flood warning", announcing that spillway gates at the Ross River dam had been opened to their maximum setting and a rapid rise in the water level was predicted to follow.

"Dangerous and high velocity flows will occur in the Ross River Sunday night into Monday. Unprecedented areas of flooding will occur in Townsville," a statement said, adding there was a "risk to life and property".

Many homes in the city had already been left 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 told reporters.

Up to 20,000 homes are 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 told reporters Saturday.

A Year's Worth Of Rain

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 told AFP the downpours could continue until Thursday, while floodwaters would take some time to recede even when the rains lessen.

The region receives an average of about 2,000 millimetres (6.5 feet) 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 deep.

"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."

Blazak said that with adverse weather predicted to continue for up to 72 hours, some regions could see record-breaking levels of rainfall.

There has been a silver lining to the deluge, with drought-stricken farmers in western Queensland boosted by the soaking.

"It is a welcome relief, especially in our western communities, to not only get the rain but also to fill up their dams," Palaszczuk said on Sunday.

"We're getting food supplies in there. We still have many roads that are cut around those areas."

The deluge comes amid a severe drought in the eastern inland of the vast Australian continent, including parts of Queensland state, that has left graziers struggling to stay afloat.

Extreme heatwaves during the southern hemisphere summer have led to maximum-temperature records being broken in some towns.

High temperatures are not unusual in Australia during its arid southern hemisphere summer, with bushfires a common occurrence.

But scientists say climate change has pushed up land and sea temperatures and led to more extremely hot days and severe fire seasons.

In the southern states of Victoria and Tasmania, firefighters in recent days have been battling numerous bushfires threatening homes and communities.