Five people died as floodwaters swept across northern New South Wales, a river-braided region about 400 kilometres (250 miles) from Sydney.

"My wife and I have been so greatly concerned to hear of the widespread and devastating flooding," said Charles, Australia's head of state.

"As the immediate emergency passes, I am only too aware that communities are confronting dreadful, soul-destroying damage to homes, properties and infrastructure, and the loss of precious livelihoods and livestock," he said, according to a statement published by Governor-General Sam Mostyn on social media.

"I am deeply conscious that the impact of the crisis will endure for many months."

The storms dumped more than six months' worth of rain over three days, according to the government weather bureau, smashing records in some areas.

The rising tides briefly stranded 50,000 people, forcing some to clamber atop cars, houses and highway bridges before helicopters winched them away.

Some 800 houses have been rendered "uninhabitable", authorities said, and thousands more properties suffered damage.