Five people have been swept to their deaths by flash flooding in northeastern Australia, officials said Saturday, after what the Queensland state premier described as an "off-the-scale" deluge.

State Premier Annastacia Palaszczuk had warned that the weather bureau had forecast extremely heavy weather for southeast Queensland on Friday afternoon.

But in announcing the first three deaths late Friday, Palaszczuk said that a deluge of 277 millimetres (almost 11 inches) of rain fell in the area where the deaths occurred in just three hours.

"This is off the scale," she said.

Police said four of the deaths occurred when two 4-wheel drive vehicles were swept away from the same bridge in Caboolture, some 44 kilometres (27 miles) north of Brisbane on Friday afternoon.

In the first accident, a 74-year-old man, a 39-year-old woman and a five-year-old boy all died.

In the second, two passengers -- a 21-year-old woman and a boy aged 16 -- managed to escape. The body of the driver, a 49-year-old man, was found early on Saturday.

In a third fatal incident involving vehicles being swept away, a 75-year-old man died in nearby Burpengary but police were able to rescue a 68-year-old woman found clinging to a tree.

"We are devastated," said Queensland's Police, Fire and Emergency Minister Jo-Ann Miller as she revealed that authorities had received some 4,000 calls for assistance over a five-hour period on Friday afternoon.

Almost 60 swift water rescues were conducted as the storm hit, she said.

- 'Well and truly flooded' -

But police warned again of the dangers of driving into floodwaters.

"It's unfortunate that with events like this, we again need to highlight to the community the dangers of entering flooded road waters. If it's flooded, forget it," Queensland Police Inspector Lee Jeffries said.

"The reports that I have is that the roads were well-and-truly flooded at the time and vehicles have entered flooded roads."

Southeast Queensland was lashed by the storm, which forced the postponement of a rugby league Test between Australia and New Zealand in Brisbane.

Torrential rains closed roads, prompted dozens of rescues and cut power to thousands of homes.

Prime Minister Tony Abbott said the federal government would assist Queensland in the wake of the deadly storms.

"Obviously our thoughts and our prayers are with the families of those who have suffered," he said.

The weather bureau said Saturday that the worst of the storm was over for Queensland, and the weather was weakening as it moved south into New South Wales state.

"Apart from still some pretty wild surf conditions along the coast, apart from that the event is pretty much over for us in southeast Queensland," Bureau of Meteorology spokesman Jonty Hall told the Australian Broadcasting Corporation.

New South Wales was hit by wild weather last month that left four people dead and caused millions of dollars in damage across Sydney and surrounding areas.

In that event, relentless gale force winds and torrential rain brought down trees and power lines, many crushing houses and cars, with a handful of homes washed away in what was described as a once-in-a-decade storm.