Australia's "once-in-a-century" floods have brought crocodiles out on the streets.

Australia's military has been deployed to tackle devastating "once-in-a-century" floods that have inundated homes, schools and airports in the country's northeast, forcing hundreds to flee and bringing crocodiles onto the streets.

The Australian Defence Forces filled sandbags, deployed amphibious cargo vehicles and helped pluck flashlight-wielding residents from their rooftops Monday, as monsoon rains drenched the northern state of Queensland.

Australia's tropical north typically experiences heavy rains during the monsoon season, but the recent downpour has far exceeded normal levels.

The authorities were forced to open floodgates late Sunday, unleashing what they called "dangerous and high velocity flows."

In hard-hit Townsville cars were mostly submerged, with picket fences barely poking through waist-deep flood waters.

"We've never seen so much water in our lives," said local radio journalist Gabi Elgood. "You think there can't possibly be any more to come but the rain just doesn't stop."

Desperate residents had to contend not only with flash flooding, landslides and power blackouts, but also reptilian predators that have been spotted in residential roads and cul-de-sacs.

There were several saltwater crocodile sightings in the flood-ravaged Townsville area.

Emergency services have struggled to respond to the scale of the disaster, carrying out 18 "swift water rescues" overnight.

More than 1,100 people have called the emergency services for urgent help, according to state premier Annastacia Palaszczuk.

Around 400 Townsville residents have sought shelter at nearby Lavarak military barracks and the Red Cross is also assisting with the response and recovery effort.

"Small boats worked through the night to evacuate members of the community," said local commander Brigadier Scott Winter.

Palaszczuk warned the communities face more difficulties ahead. Schools and courts remain closed, more rain is on the way and emergency warnings still in effect for more than a dozen rivers.

Ergon Energy's spokesperson Emma Oliveri told the AFP that more than 16,000 people were without power and they are unable to say when the lights will come back on.

The town of Ingham, just north of Townsville, got over 10 centimetres of rain in just a few hours on Monday morning, according to the Bureau of Meteorology.

Winds are expected to gust at up to 100 kilometres (62 miles) an hour on the coast.

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

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

"This is unprecedented, we've never seen anything like this before," she said.

A year's worth of rain

The Bureau of Meteorology's Adam Blazak told AFP the downpours could continue until Thursday, while floodwaters will take some time to recede even when the rains lessen.

Some areas are expected to get a year's worth of rain in just over a week.

The region receives an average of about 2,000 millimetres (80 inches) of rain annually, but some towns are already on track to pass that.

"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.

A silver lining to the deluge is that it has boosted drought-stricken farmers in western Queensland.

The downpours come amid a severe drought that has left graziers struggling to keep in business.