Thousands of residents of Australia's third-largest city evacuated homes on Wednesday as massive floods began to inundate the financial district, sparked panic buying of food and left authorities despairing for nearly 70 people missing.

The biggest floods in a century have so far killed 16 people since starting their march across the northern mining state of 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.

With a flood surge expected to peak on Thursday in the state capital of Brisbane, a city of two million, residents pushed food-laden shopping carts through drowned streets as supermarkets were stripped of milk and bread staples.

At one business center, people waded in shoulder-high water trying to rescue possessions, while on the nearby riverside boats and pontoons were ripped from moorings, crashing into bridges as the muddy brown tide gathered in 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.

In the nearby city of Ipswich, which will be hit by the flood peak in the next few hours, 3,000 homes were already 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.

Rescue crews took advantage of some rare sunshine to look for 67 people still missing from tsunami-like flash floods that tore through townships west of the city this week.

We can take no comfort from that blue sky, Queensland state Premier Anna Bligh told reporters, predicting almost 20,000 homes could be flooded at the river's peak in what she called Queensland's worst natural disaster.

The water and the rain have already done their damage. This is a deeply serious natural disaster.

The worsening floods are forcing economists to raise estimates of the economic impact, with one central bank board member on Wednesday saying the disaster could cut 1 percent off growth -- equal to almost $13 billion, double the previous highest estimate.

The Australian dollar sank to a fresh four-week low of $0.9803 on the comments from Warwick McKibbin, an academic and a member of the central bank's policymaking board.

Treasurer Wayne Swan in November forecast GDP growth of 3.25 percent in fiscal 2010-11, up from a 3.0 percent projection, but said spending would be cut to ensure a surplus of A$3.1 billion or 0.2 percent of GDP in 2012/13.

Food prices are surging around the country as the floods ruin crops in Queensland and sever distribution networks.

In Brisbane, thousands of homes and businesses were deserted as swirling floodwaters rose in and around the riverside city, forcing residents to flee with few possessions to higher ground and to evacuation centers that were crowded with more than 3,500 people.

Some of the scenes in the city were surreal, with early-morning joggers trying to carry on as normal, even though parts of their routes were underwater. Others were distraught.

This is my whole life, everything is gone. I never thought it would get this bad, said Kim Hung, manager of the Salt 'n' Pepper catering business, as two friends floated a coffee machine toward higher ground.

Bligh said she expected about 19,700 homes to be flooded at the river's peak, affecting up to 45,000 people. The military is running relief flights with helicopters and C-130 transports.

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 5-million-tonnes-a-year 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. Some 78,000 homes in the southeast of Queensland were without electricity.

'Terrfying, Chaos'

Showers are forecast to return next week, and state Premier Bligh said the peak flood levels in Brisbane may last for days. She advised people not to leave their homes if they are safe and to conserve water in case fresh supplies were interrupted.

As crazy as it sounds, now is not a time to be wasting water, Bligh said.

Prime Minister Julia Gillard arrived in the city to inspect the devastation and said she was deeply concerned about the impact on jobs and livelihoods.

I have been shocked. I think we've all been shocked by the images of that wall of water just wreaking such devastation. The dimensions of it are truly mind-boggling, Gillard said.

The floods have been blamed on a La Nina weather pattern in the Pacific, with Australia recording its third-wettest year on record in 2010, with two wet-season months to go. Weather officials are also forecasting an above-average cyclone season.

As the Queensland floods and rains move south, major rivers in New South Wales state have begun flooding or threatening to break their banks, forcing rural evacuations. In southeast Victoria state, heavy rain caused flash flooding and landslides, prompting fears a lake near Horsham could break its banks.

However, underlining the country's summer of extremes, authorities in Western Australia battled huge bushfires.