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Brisbane, Australia (CNN) -- Brisbane was warned that a wave of water was sweeping through the city's main river system on Wednesday, threatening to exceed the damage done by the record 1974 floods.
The Brisbane River, which cuts through the city center, was expected to rise another 2 meters on Wednesday before peaking at 4 a.m. Thursday at more than five meters.
"We are preparing for the worst natural disaster in our history," said Queensland Premier Anna Bligh. Models predict that 19,700 homes will be "completely flooded" in Brisbane, which is home to more than 1 million residents and another 4,000 homes are likely to be affected in Ipswich, she said.
The river is expected to rise beyond the records set in 1974 when it broke its banks, flooding more than 6,000 homes and killing 14 people.
The devastating impact has not been limited to flooding. Some places that remain dry have been isolated for more than two weeks, she said. Medicines and other supplies have been ferried in by helicopter in some cases. "These floods have an effect, whether they're in your backyard and coming through your floorboards or whether you are isolated," she said.
Twelve people have died and 67 were missing, Queensland Premier Anna Bligh told reporters in Brisbane.
Twenty-five helicopters were involved in operations in Toowoomba and Brisbane, she said Wednesday.
Most were missing in and around the city of Toowoomba, which was devastated Monday by what is being described as an "inland tsunami." Toowoomba is about 100 kilometers west of Brisbane.
"I think we've all been shocked by the images of that wall of water just wreaking such devastation and.. when we hear the statistics about how many homes are going to be hit in Ipswich and here in Brisbane, the dimensions of it are truly mind-boggling," Australian Prime Minister Julia Gillard said.
"It is terribly sad and I do fear that as a nation we have to prepare ourselves for some more bad news and for a rising death toll," she told Sky News. "The nation is going to have some grieving to do and today we've got some work to do as well, to assist people to prepare and to keep with the search and rescue, there are Australians desperately waiting for news."
She told 4BC Brisbane that she has received offers of help from around the world.
Residents spent Tuesday sandbagging their homes and moving valuables to higher ground and, in some cases, evacuating their homes.
Steve Cook, a resident of the Brisbane inner suburb of West End, spent the afternoon loading his family's belongings into vehicles to be driven away from the flood zone.
"It's pretty chaotic on the streets at the moment," he said. "The streets are almost blocked with four-wheel drives and utilities and everyone is desperately packing their belongings into cars to try to get away before the flood peaks."
By late Tuesday, the water was a meter (3 feet) from his doorstep, one street from the banks of the Brisbane River. Like thousands of others here, he was moving his wife and two children to a relative's house on higher ground.
Onlookers gathered Tuesday at the bank of the Brisbane River as the waters rose. Debris could be seen floating in the fast-flowing water.
"We're just watching the river now and we've seen jetties come past, all sorts of things just floating down the river," said Desire Gralton, who lives in the ground-floor apartment of a four-story building in the Brisbane suburb of New Farm on the Brisbane River. "It's amazing."
She and her family their personal belongings, photographs and documents to their car outside after stacking their belongings in order of importance -- the most valuable items piled atop furniture they were resigned to losing.
"We're expecting it to come through about one meter through our apartment, so we're trying to move everything higher than that and hope for the best," she said. "If the flood comes into our level, we'll all lose our electricity. So pretty much everyone would have to be evacuated."
Power was being switched off Wednesday to homes in Brisbane and Ipswich, west of the city, as flood water threatened electricity sources. Around 21,000 homes were without power in Ipswich, as well as up to 4,000 homes in Brisbane where water had already breached the banks of the Brisbane River.
"We've already started isolation of the low-lying areas" of Brisbane's Central Business District, said Energex spokesman Mike Swanston. By the predicted river peak on Thursday afternoon, power cuts could extend to about 10 city blocks, he said.
Hundreds of residents sought shelter Tuesday night in evacuation centers in Brisbane and Ipswich where they were expected to stay for days until the water recedes.
The heavy rain and predicted flooding in Brisbane follows flooding elsewhere in the state. Soon after Christmas, water swamped the northern city of Rockhampton, 600 kilometers (370 miles) north of Brisbane, triggering a massive recovery operation to rebuild the town.
In a development that Bligh acknowledged "seems completely ridiculous," the premier pleaded with residents of Brisbane to conserve water, since the city's treatment plants could also be jeopardized by the floodwaters. "As crazy as it sounds, now is not a time to be wasting water, because we don't know what might happen with our water supply," she said.
CNN's John Raedler contributed to this story