A five-year-old boy is feared to have been snatched by a crocodile in floodwaters in northern Australia while walking with his dog on Sunday, police said.

"The boy was walking with his seven-year-old brother earlier this morning when he followed his dog into floodwaters," police said in a statement.

"He disappeared in the water and his brother saw a large crocodile in the vicinity of his disappearance."

A large-scale search for the boy has been launched at Cape Tribulation in far north Queensland.

Police were also searching for two people missing after their car was washed away as they tried to drive through floodwaters south of Tully in Queensland.

Much of the state has been declared a disaster zone, with an area of more than a million square kilometres (386,100 square miles) and 3,000 homes affected by floods due to torrential rains.

Hundreds of people were forced to evacuate and others have been stranded in their homes.

Rain continued to fall on the saturated state on Sunday, easing in the worst-hit north but threatening to flood areas that have so far escaped the monsoonal downpours.

"Tomorrow, the monsoon trough continues its movements south and the bulk of the rain will head down towards Rockhampton," a weather bureau spokesman said.

"We can expect local flooding to occur as that rain moves south."

While the north of eastern Australia has been swamped, the south has faced scorching heat and deadly wildfires.

The death toll from bushfires raging through Victoria state soared to 35 Sunday and was set to mount in a disaster Prime Minister Kevin Rudd described as "hell in all its fury."

Most of those killed were caught up in an inferno northwest of Melbourne, where entire townships had been razed in the firestorm, which continued to burn out of control Sunday.