Residents of 20 suburbs in Townsville in Queensland have been told to get to higher ground, with the council saying dam spillway gates may have to be fully opened, starting in about half an hour.
Opening the spillway would release up to 2000 cubic metres of water per second from the Ross River Dam, over nine and a half hours overnight.
Authorities say that would potentially cause rapid flooding across the city.
Townsville has set a new seven-day rainfall record from monsoonal rain hitting north and northwest Queensland.
The city council says residents who won't leave need to go to the highest point in their houses. Affected suburbs would include Rosslea, Hermit Park, Railway Estate, Townsville City, Oonoonba, Idalia, Cluden, West End, Rowes Bay, Garbutt, Aitkenvale, Cranbrook, Currajong, Mysterton, Pimlico, Mundingburra, Douglas, Annandale, Kirwan and Thuringowa Central and South Townsville areas.
Bureau of Meteorology (BOM) forecaster Dean Narramore said this morning that there had been some extraordinary rainfall numbers coming in for the Townsville area, with a number of locations receiving over a metre of rain in the last seven days.
"Particularly a number of locations to the west of Townsville where we've seen 1.4 to 1.6 metres of rain - for some locations this is approaching their yearly average in just a week," Mr Narramore said.
Earlier today the bureau said tornado-strength winds were possible along the north Queensland coast during the day.
Townsville Mayor Jenny Hill said about 1000 cubic metres of water a second was still going over the Ross River Dam spillway.
By midday today, the dam had risen above 231 per cent capacity, while a severe storm warning noted "intense" rain falling over the catchment.
"We don't know how much rain is still likely to flow into the Ross River Dam catchment," Cr Hill said.
The army has delivered 90,000 sandbags to affected Townsville homes.
Townsville disaster coordinator Assistant Chief Superintendent Steve Munro said there were 82,000 houses in Townsville and flood modelling predicted up to a quarter would be affected by flooding in the worst-case scenario.
"At the moment, we have probably got 400, 500 [homes] affected [by flooding]," he said. "The modelling says what it is going to say - it could move up to the 10,000, 20,000 [homes]. We don't want to get to that stage [but] we can't control that."
Earlier, Townsville's flood emergency was declared a catastrophe by the Insurance Council of Australia (ICA).