A teenage boy was dramatically rescued from surging floodwaters as he clung to a tree Friday, while others were plucked from stranded cars and homes as floods swept northeastern Australia.

Authorities said there were 20 water rescue cases across central Queensland state overnight and early Friday, after the remnants of tropical cyclone Oswald dumped huge rains around the coastal city of Rockhampton.

Rescues included those of a woman and two children trapped in a car isolated by flood waters, and seven people taken from two flooded homes.

Authorities said a teenage boy was lucky to be alive after he was found clinging to a tree as muddy floodwaters surged around him at Frenchville, some 525 kilometres (325 miles) northwest of Brisbane, early on Friday.

"The current was so strong it just took him away," station officer Brett Williams told the Australian Broadcasting Corporation.

The national broadcaster said the boy's rescuer was himself swept under the waters after helping the teenager to dry land, but later bobbed up safely further down the swollen creek.

The Bureau of Meteorology issued a severe weather warning, saying with ex-tropical cyclone Oswald over central Queensland, damaging winds with gusts of up to 90 kilometres per hour (55 mph) were possible in the wider Rockhampton area.

The torrential rain has flooded rivers and dumped enough water to isolate towns, with Rockhampton experiencing its biggest daily downpour since 1939.

The bureau said severe flash flooding was reported overnight in the Yeppoon area, north of Rockhampton, where rainfall of 271 millimetres has been recorded since early Thursday.

In Gladstone, south of Rockhampton, the town was cut off by the flood waters, but damage was limited.

"It's very wet here," Gladstone regional mayor Gail Sellers told ABC TV.

"It hasn't been cyclonic as such but the winds are very strong."

Oswald has also impacted communities in far north Queensland and as the weather system makes its way south, heavy rainfall is expected for the next few days even as Rockhampton moves into clean-up mode as the waters there recede.

"We will be working now to clear up and we ask people to be careful where roads are still covered, where there are powerlines down," Rockhampton Mayor Margaret Strelow said.

The rain has left major highways closed, disrupted rail services and placed Emergency Management Queensland on high alert.

Two years ago floods in Queensland left more than 30 people dead, 29,000 homes and businesses inundated and some 2.5 million people affected.