Just after 8pm last night, the spillway gates of the Ross River Dam opened and flash flooding warnings were sent out to many suburbs.
Northern Queensland usually experiences a tropical climate at this time of year.
The pair had not been located by Monday night despite extensive inquiries with family and friends, and a search of floodwaters and the area has begun as a precaution, police say.
It is believed 424 residents are at Lavarack Barracks this morning in a temporary evacuation centre, with helicopters and army personnel preparing to head out to assess the damage and evacuate further people needing help.
The national broadcaster ABC reported that thousands of homes had been abandoned in the suburbs of Townsville and that evacuations were being conducted.
The premier said about 200 callers were still waiting for help on Monday morning.
Schools in Townsville remain closed and a decision will be made early on Monday about whether to reopen the city's airport after all flights were cancelled late on Sunday.
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.
"In Queensland, of course, we're used to seeing natural disasters, but Townsville has never seen the likes of this".
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Premier Annastacia Palaszczuk said the severe weather would move south, putting Mackay in the warning area. Nathan Hughes via REUTERS Ross River Dam releases water in Queensland, Australia, in this still photo from a February 3, 2019 video by Julia Hunt.
The north-east Australian city of Townsville could face tornadoes, just as it is being hit by devastating floods, authorities said Sunday.
Desperate residents had to contend not only with flash flooding, landslides and power blackouts, but also several saltwater crocodile sightings in residential roads and cul-de-sacs.
Emergency services are fearful anywhere between 10,000 and 20,000 homes could be inundated as the floodwaters continue to rise and predictions of more heavy rain in the coming days.
With the dam at more than 300 percent capacity, engineers said they had no option.
Blazak said that with adverse weather predicted to continue for up to 72 hours, some regions could see record-breaking levels of rainfall.
"It's basically not just a one-in-20-year event, it's a one-in-100-year event", she said.
"We were lambs to the slaughter", says Mr Roberts who lives in low-lying Hermit Park.