Massive floods shut down the center of Australia's third-largest city, sent thousands fleeing from their homes and sparked panic buying of food on Wednesday as rescuers searched for 43 people missing in floodwaters.

Australia's biggest floods in a century have so far killed 16 people since starting their onslaught across northern mining state Queensland last month, crippling the coking coal industry, destroying infrastructure, putting a brake on the economy and sending the local currency to four-week lows.

The flood surge is expected to peak in Brisbane, a riverside city of 2 million people, before sunrise on Thursday and last for days.

"We are in the grip of a very serious natural disaster," Queensland state Premier Anna Bligh said, predicting almost 20,000 homes could be flooded at the river's peak."Brisbane will go to sleep tonight and wake up to scenes many will never have seen before in their lives," she warned.

The flood peak hit Ipswich, a satellite town to the west, late on Wednesday. More than 1,500 Ipswich residents sheltered in evacuation centers, but others fled homes with little more than what they are wearing, as floodwaters rose around the city.

Ipswich Mayor Paul Pisasale said the flood peaked at 19.4 meters, about a meter below expectations, saving some 6,000 homes from flooding. "It's the difference between bad news and devastation," he said.

Brisbane residents on Wednesday pushed food-laden shopping carts through submerged streets, others waded in shoulder-high water to rescue possessions, while boats and pontoons were ripped from moorings in the Brisbane River and smashed into bridges as the muddy brown tide gathered strength.

At flooded intersections people paddled surfboards through floodwaters, balancing their possessions on the deck of the boards, while boats ferried evacuees to dry ground.

"I am feeling a sense of horror and awe at the power of the river. Sadly in coming hours we will see bits of people's homes float down the river," Brisbane Mayor Campbell Newman said, warning the torrent could take three to four days to subside.

Four thousand people sheltered in evacuation centers.

Economists have raised their estimates of the economic impact of the flooding, with one central bank board member saying on Wednesday the disaster could cut 1 percent off growth - equal to almost $13 billion, double the previous highest estimate.

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

Food prices are surging around the country as the floods ruin Queensland crops and distribution networks. Prices for tomatoes have leapt about 200 percent in two weeks, while beef is up 11 percent and wheat has risen 4 percent in four months.

Rescue crews took advantage of some rare sunshine to look for 43 people still missing from tsunami-like flash floods that tore through townships west of the city this week.

"Some of the houses have exploded with the force of the water. People have been washed away. It will take days to complete this search and recovery," said Deputy Police Commissioner Ian Stewart.