Rainfall poured down by ex-tropical cyclone Oswald has brought flowing to the eastern state of Queensland, Australia, where so far 20 have been rescued.

The Bureau of Meteorology has warned the state it faces a 75 per cent chance of major flooding this weekend, from along the coast from the Queensland border to Taree.

Ex-tropical cyclone Oswald poured enormous rainfall around the coastal city of Rockhampton, prompting authorities to conduct 20 water rescue cases across central Queensland overnight and early Friday alone. The rescue efforts included a woman and two children trapped in a car isolated by flood waters, seven people plucked from two flooded homes, as well as a teenage boy.

The 14-year old boy was found early on Friday clinging for his life to a tree as muddy floodwaters surged around him at Frenchville, some 525 kilometres (325 miles) northwest of Brisbane.

Bill Ludwig, Rockhampton councilor, advised residents to stay off the streets and to keep warm.

"They're saying this is not a cyclone but it's not far off it," he said.

Ex-tropical cyclone Oswald, now about 120 kilometres west of Mackay, with the amount of rainfall it has dumped, has left many towns isolated because the severe flooding has cut many roads off. It is expected to continue giving off heavy rains as it moves south over the weekend.

Rainfalls predicted for some areas could come in at more than 400mm, along with damaging winds of around 90kph.