Sharks in the streets of Brisbane

• Anne Smart sits on the steps of her flooded house yesterday after water the day before inundated the city of Ipswich, 40km from Brisbane. Picture: AFP/Getty Images

The state capital of Brisbane is facing its worst devastation in more than 100 years as high tides and heavy rain combine to leave some streets under 15ft of water.

Residents woke up to the worst of the devastation yesterday as emergency services battled to bring under control a situation which has already claimed 21 lives in Queensland.

Australian Prime Minister Julia Gillard called the disaster's scale "mind-boggling" and warned that the death toll will rise.

Queensland's premier Anna Bligh warned: "We are now in the grip of a very serious natural disaster.

"We are now seeing thousands of homes inundated with water up to the roof. Many, many more are expected to see significant water damage."

She said 20,000 to 30,000 people would be affected in Brisbane.

Although the flood peak could be below the 1974 level, Ms Bligh added: "This is still a major event. The city is much bigger, much more populated and has many parts underwater."

Unprecedented rainfall over much of the north-east Australian state has already devastated scores of towns and cities.

The surging waters in Brisbane's empty city centre reached the tops of traffic lights and have left at least 20,000 homes in danger of being inundated.

Debbie Reilly, who emigrated from Scotland to Brisbane five years ago, said there were reports of "bull sharks swimming through the water" and fears of E coli in the water.

Ms Reilly, originally from Aberdeen, said: "We can't even help people because we've been told to stay where we are.

"It's been raining for three weeks. We're still in danger because four more cyclones are expected."

The 27-year-old waitress and her boyfriend Shane, 44, live in a third-floor apartment less than a quarter of a mile from the flooding. She added: "Everywhere is closed. I went to the shop and there was no bread left, no vegetables and limited milk. There were just three tins of tomato soup."

• 'Queensland is going to need a lot of help later'

Residents were forced to take cover on higher ground, while others scrambled to move their prized possessions to the top floor of their homes.

The crisis escalated when a violent storm sent a 26ft-high, fast-moving torrent - described as an "inland instant tsunami" - crashing through the city of Toowoomba and smaller towns to the west of Brisbane on Monday.

Twelve people were killed in that flash flood, with 43 missing.

"This is a truly dire set of circumstances," Ms Gillard said. "People do have to be prepared for the death toll to rise."

Brisbane resident Rob Minshull said a bull shark had been spotted swimming in the streets. Hundreds of bull sharks, which can live in fresh or salt water, inhabit Brisbane River.

Water levels were expected to stay at peak levels until at least Saturday, but many people will not be able to reach their homes for several days beyond that.

The flooding is shaping up to become the nation's most expensive disaster, with an estimated price tag of at least five billion Australian dollars. The relentless waters have shut down Queensland state's coal industry and ruined crops across vast swathes of farmland.

Boats torn from their moorings floated down the rising river along with massive amounts of debris. A popular waterside restaurant's pontoon was swept away by the current and floated downstream.

Officials opened three more evacuation centres, creating room for 16,000 people to take shelter.

The city's main power company said it would switch off electricity to some parts of the city as a precaution against electrocution. Almost 70,000 homes were without power.

"I know that this is going to be very difficult for people," Ms Bligh said. "Can I just stress: electricity and water do not mix. We would have catastrophic situations if we didn't shut down power."

For weeks the flooding had been a slow-motion disaster, devastating farmland and towns.

On Monday the crisis took a sudden, violent turn, with a cloudburst sending a raging torrent down the Lockyer Valley west of Brisbane.

Houses were washed from their foundations and cars tossed about like toys in what police commissioner Bob Atkinson described as an "instant tsunami".

Hundreds had to be rescued by helicopter, and emergency vehicles were moving into the worst-hit parts of the valley.