After years of unending drought, Australia's wheat belt has been pounded by torrid rains - and a giant swarm of locusts - forcing mass evacuations and wiping millions off the nation's economy.

New South Wales, Australia's key economic province, has been declared a natural disaster zone after floods, driven by torrential rain, threaten the nation's agricultural economy and drive up global wheat costs.

Flood waters are expected to peak in the next 24 hours, driving thousands from their homes and farms at a critical time for farmers already devastated after years of suffocating drought.

"It's like the hand of heaven has just reached down and swept us away", said Gundagai resident Rob Butcher from his mobile after watching an entire wheat crop destroyed by rising flood waters in the last 36 hours.

"I don't know if we can get back up after this one," he said.

Almost the entire region, from Gundagai to Wagga in central NSW has been evacuated from their homes as the New South Wales state Premier Kristina Keneally declared six more shires as natural disaster zones, bringing the total in NSW to 34.

While Keneally told journalists that natural disaster declarations for the Upper Lachlan, Wagga Wagga, Parkes, Narromine, Gilgandra and Warren would allow fast assistance to those regions inundated, the NSW Emergency Services Minister Steve Whan warned of the dangers of attempting to cross flood waters.

"It is absolutely imperative (not to cross waters) ...that is very dangerous and can cause deaths," he said.

A particularly strong La Nina meteorological event is hammering Australia's east coast. Torrential and consistent rainfall, caused by the cooling the Pacific Ocean and has produced the wettest August-to-October period on record, according to figures recorded by the Bureau of Meteorology.

According to local farmers, the government has "vastly" underestimated the potential damage to the key agriculture economy, and the flow-on impact to the wider economy.

Farmers representatives led by the president of the NSW Farmers Association, Charles Armstrong, told reporters that more than 1 billion AU dollars could be wiped off the value of the state's crops. This is double the estimated government predictions.

In a statement released on Monday, New South Wales Emergency Services Minister Steven Whan maintained that only half a billion AU dollars would be wiped off the state's forecast bumper 3.2 billion AU dollars winter harvest.

Rob Butcher said that the NSW regional wheat crop was almost certainly destroyed. He told Xinhua that he expected that more than half of this year's giant NSW wheat crop would be " unrecoverable."

To add salt to the wounds, wheat futures on the Chicago Board of Trade Sunday were up 13 percent for the week, the most since early August.

Prices are expected to keep rising as the damage from the floods may be compounded by reports of a massive swarm of locusts heading through southern NSW toward northern Victoria.

Flood warnings have also been issued for rivers in the northern state of Queensland and the southern state of Victoria.