A man paddles his kayak next to a submerged bus on a flooded street in the town of Milton, Brisbane, on Monday. AFP

Tens of thousands of people were ordered to evacuate as heavy rains smashed Australia's east coast on Monday, submerging towns and stranding residents on rooftops, with authorities warning of life-threatening flash floods.

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.

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

This handout photo shows a view of the south-east Queensland. AFP

"We were rescued by two guys in a boat, two locals," Hardiman told the media, 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."

Nine people have been killed since the deluge began last Thursday, and rescue teams were searching on Monday for at least four people reported missing.

People use a small boat to travel through floodwaters in Lismore, Australia, on Monday. AP

Prime Minister Scott Morrison, who called the unprecedented rains a "weather bomb", said defence personnel would be deployed to flood-hit areas to lead both rescue and recovery operations.

Australia's weather bureau said flash flooding remained a real risk in northern New South Wales (NSW) state as the wild weather moved south from neighbouring Queensland.

"What we are seeing today is unprecedented and the advice that we have received is we would expect things to get worse," NSW Premier Dominic Perrottet told a televised briefing.

A rescue crew moves through the flooded streets in the city of Paddington in suburban Brisbane on Monday. AFP

More than 400 millimetres (16 inches) of rain has fallen in the past 24 hours around Lismore, with the town's Wilson River still rising, according to the weather bureau.

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

The Brisbane River in the Queensland capital peaked on Monday morning, flooding several streets in Australia's third largest city. Social media posts showed indundated houses and debris, including bins, boats and cars, floating down roads.