Tens of thousands of Townsville properties are at risk of flooding after a heavy deluge of rain forced the opening of the Ross River Dam floodgates for the first time since 2014.

Authorities opened the spillway gates late on Sunday night after the dam reached almost 250 per cent capacity. That released about 2000 cubic metres of water a second out of the dam.

The extra water is threatening to inundate up to 20,000 Townsville homes as residents in low-lying suburbs are being warned to seek higher ground.

Two police officers were caught in the deluge as they helped to evacuate people in Hermit Park after the floodgates were fully opened.

Their vehicle was washed away and they were left clinging to trees for about half an hour before a swift-water rescue crew arrived.

Crews also rescued two people trapped on the roof of their car in nearby Hyde Park.

Premier Annastacia Palaszczuk said several thousand homes might have been inundated overnight on Sunday, but said that did not necessarily mean they had water over their floorboards.

“They’ll do the damage assessment but that could be the case because some houses are high set,” Ms Palaszczuk said.

In addition to the dam water, residents can expect another 450 millimetres of rain to fall over the dam catchment by the end of Monday, according to the Bureau of Meteorology.

BOM hydrologist Andrew Preece said the opening of the dam gates posed an unprecedented threat to Townsville homes.

“What that means is we’ll see dangerous and high-velocity flows in Ross River, resulting in risk to life and property,” he said.

“We’ll see unprecedented areas of flooding in Townsville, a number of access routes cut, over the next 24 hours or so.”

Further six hour totals from 150-200 millimetres are likely to continue as well, along with wind gusts of up to 100km/h.

Townsville Mayor Jenny Hill said the dam gates, which operate automatically, were forced open. There was nothing operators could do to seal them shut.

“Sadly, we received a huge burst of rain at about 1.30 this afternoon for about three hours, that just tipped us over,” she said.

“We thought we’d been able to manage it, but the rainfall we had, there was nothing we could do.”

A week of relentless rain has brought 20 times Townsville’s average rainfall – the worst since the city’s so-called Night of Noah in 1998.

Power has been cut to thousands of homes in the north Queensland city, and hundreds of people who’ve been forced from their homes by waist- and chest-deep water are taking refuge at evacuation centres, and with family and friends on higher ground.

Ms Palaszczuk said the state’s flood disaster was far from over. Weather forecasters have said between half a metre and a metre of extra rain could fall on the east coast in coming days.

A severe weather warning covers communities from Mackay, on the coast, north to Ingham, and right out west, almost as far as the Northern Territory border.

Major road routes are cut, including the Bruce Highway north and south of Townsville, forcing the state government to shore up food supplies in isolated communities, including Richmond.

Eleven of the state’s local government areas have been affected, with five eligible for disaster assistance.

“We have had so many of these natural disasters over a short period of time,” Ms Palaszczuk said on Sunday, referring to earlier floods and cyclones during summer.

“The flooding event is going to cause a lot of distress to a lot of people.”

In Townsville, authorities have warned people that even when the rain stops, it could take days for the water to subside so they can go home.

“We are literally in uncharted territory for this city,” Ms Hill said.

“The real test will be once we hit recovery.”

-with AAP