Authorities in the Australian city of Brisbane are urging citizens to evacuate as flood waters threaten low lying areas.

As many as 70 people have been reported missing in the state of Queensland, with nine confirmed dead.

Flood waters have been sweeping the state for three weeks, with 200,000 people affected.

Tsunami conditions are expected to hit the city over the next 48 hours, with police urging residents in low-lying areas of the riverside city to move to higher ground.

“Major flood levels will develop during Wednesday and it will continue rise into Thursday,” added a spokeswoman for the Bureau of Meteorology.

The Brisbane River snakes its way through the centre of the Queensland state capital, and in places it has burst its banks already.

Lower lying suburbs – including the West End - have already been inundated by water.

Sandbags are being distributed to the two million strong population, with city mayor Campbell Newman warning 6,500 homes, businesses and other properties were likely to be flooded by Thursday.

“Today is very significant, tomorrow is bad, and Thursday is going to be devastating for the residents and businesses affected,” he explained.

Residents of Brisbane compared events to the catastrophic 1974 flooding in which 6,700 of the city’s houses were swamped and 14 lives lost.

State premier Anna Bligh urged people to prepare themselves:

“For those who are living in some of the lower lying areas and the identified suburbs, now is the time to be making whatever preparations you can, and I would encourage you to be overly cautious,” she said.

“It is better to be inconvenienced and find that your preparations were not necessary, than the alternative.”

Across Queensland flooding is now into its third week.

Waters have affected half of the state - swamping an area the size of France and Germany combined - and dislocated the economically vital coal industry.

Harvests of wheat, sugar cane and cotton have also been wrecked, while the infrastructure repair bill is conservatively estimated at US4.9 billion.