Tens of thousands of Australians were ordered to flee their homes Monday, as torrential rain sent floodwaters to record levels, leaving residents stranded on the rooftops of their homes.

Seven people have died, and the country's weather bureau has warned further severe thunderstorms and intense rainfall will cause "life-threatening flash flooding" for a swathe of the central Pacific coastal region.

In the country town of Lismore, resident Danika Hardiman woke Monday morning to find mud-brown floodwaters had reached the balcony of her second-floor apartment.

She and her partner managed to climb up to the roof, where they were spotted by passing kayakers, who flagged down a makeshift rescue boat.

"We were rescued by two guys in a boat, two locals," Hardiman told AFP, describing the scenes in Lismore as "horrific".

"Imagine you're in a boat sailing past people's roofs," she said.

"The scary thing is this is just the beginning, there's lots of rain to come."

With the town's levees already breached, 43,000 residents have been ordered to leave immediately.

Emergency services were overwhelmed by calls for aid, leaving some locals -- including Lismore's mayor Steve Krieg -- to turn to social media for help.

A couple wades through a waterlogged street on the banks of the overflowing Brisbane River
A couple wades through a waterlogged street on the banks of the overflowing Brisbane River AFP / Patrick HAMILTON

"If anyone has a boat and can get to Engine Street, there's a pregnant lady sitting on her roof. HELP Please," he posted on Facebook Monday.

Emergency rescue services said they had also deployed a helicopter to pluck other stranded residents from rooftops.

Water levels in Lismore have not yet reached their expected peak of 14 metres (46 feet) -- but they are already the worst floods the town has experienced.

Across eastern Australia flooding has now killed seven people, after a 59-year-old man was swept away by strong currents just outside Brisbane.

Millions of people have been told to stay home and nearly 1,000 schools in the state of Queensland remain closed because of the floods.

A 70-year-old man miraculously survived after his houseboat, swept along by the raging Brisbane River, collided with a ferry terminal and quickly sank.

Members of the public were able to rescue the man, with one telling public broadcaster ABC they had linked arms to create a human chain and fish the man from the river unharmed.

"I don't know how he survived it, to be honest," onlooker Matthew Toomey said.

Rain began to ease in Brisbane Monday but authorities expect the severe weather to continue this week as the "rain bomb" continues to travel south along the Australian coast.