It is feared almost 20,000 homes in Brisbane are set to be swamped as river levels are expected to peak.

Massive floods have inundated the financial district, sparking panic buying of food.

Authorities are searching for nearly 70 people who are missing while the floods have so far killed 16 people.

People have been seen using paddled surfboards to get through the floodwaters, balancing their possessions on the decks, while boats ferried evacuees to dry ground.

In the nearby city of Ipswich, 3,000 homes have already been flooded and one third of the town is expected to be underwater at the peak.

"The water is rising and swallowing up the city. It's really heartbreaking," said Ipswich Mayor Paul Pisasale.

Dams built to protect communities are at bursting point. The Port of Brisbane is closed, shutting down Australia's third busiest container port and a major coal-loading facility.

Australia is the world's biggest exporter of coking coal, which is used in steel manufacturing and accounts for more than half of global exports, and is also the second-biggest exporter of thermal coal used for power generation.

Power company Energex shut power to some low-lying areas of Brisbane, including parts of the financial district, for fear that live power lines could electrify floodwaters.

Showers are forecast to return next week, while the rains are starting to affect New South Wales state threatening to break their banks and forcing rural evacuations.