The death toll from heavy rains that inundated Australia's east coast has climbed to six, as flood warnings continued Sunday following a severe deluge that cut power to thousands of homes and isolated numerous communities.

The eastern states of Queensland and New South Wales (NSW) faced a huge clean-up task ahead, but were forecast to experience easing conditions on Sunday following two days of wild weather.

"Never before have we seen such a huge volume of rain drop in such a short period of time," Queensland Premier Annastacia Palaszczuk told broadcaster Channel Seven from the state's capital Brisbane on Sunday.

A six-year-old boy became the sixth storm victim when he was swept away by rough seas at a northern NSW beach Saturday, police and surf lifesavers said.

The boy was walking along South Ballina beach with his father and older brother when he was pulled into the ocean by stormy waters, according to the surf lifesavers.

Police said he was rescued by fishermen and flown to a nearby hospital, where he died.

Five people, including a five-year-old boy, also lost their lives Friday when two 4-wheel-drive vehicles were caught in flash flooding in Caboolture some 44 kilometres (27 miles) north of Brisbane.

Regional mayor Allan Sutherland described the torrential downpour as a "one-in-2000-year event".

"To say we got smacked would be an understatement," Sutherland told Queensland's Courier Mail.

"It's surreal now. This nightmare just came through and took five souls."

Caboolture was hit by 277 millimetres (10.9 inches) of rain in just three hours Friday to take total rainfall in the area to 360 millimetres in less than 24 hours.

More than 2000 calls were made to Queensland's State Emergency Service, mostly relating to flood damage and sandbagging.

In NSW, northern towns also copped a beating, with some areas receiving more than 100 or 200 millimetres of rain over a two-day period, bureau forecaster Sarah Chadwick told AFP.

Conditions were expected to ease later Sunday and on Monday, although flood warnings remained for several regions across the two states.

Prime Minister Tony Abbott said Saturday the federal government would assist Queensland following the deadly storms.

The intense torrent of rain came just days after NSW was hit by gale-force winds and heavy downpours in a once-in-a-decade storm that left four people dead.

Several houses were swept away and powerlines and trees were brought down, crushing homes and cars, with the destruction causing millions of dollars in damage across Sydney and surrounding regions.