Brisbane told to get set for biggest soak since 1974

FORECASTERS are warning Brisbane residents to prepare for the city's biggest flood since 1974.

FORECASTERS are warning Brisbane residents to prepare for the city's biggest flood since 1974.

More than 300mm of rain fell in parts of the Brisbane River catchment in the 24 hours to yesterday afternoon.

The weather bureau released a flood warning yesterday predicting the Brisbane River would reach 2.1m this afternoon and 3m at high tide tomorrow -- double the normal expected height of the annual highest tide.

The warnings come after extreme rainfalls in eastern Queensland on Sunday night and yesterday. Bureau of Meteorology flood forecaster Jeff Perkins said the flood risk was unusual. "It's a very significant flood in that Brisbane doesn't get many floods," he said. "We are predicting the biggest flood since 1974."

Mr Perkins said the bureau was not predicting a peak level for the Brisbane River at this stage.

Rainfall gauges from Brisbane to Noosa and as far inland as Toowoomba clocked between 100mm and 300mm in the 24 hours to last night.

Mr Perkins said the water was moving rapidly downstream.

"The rainfall caused an 8m rise in just over an hour at Helidon, and that peak is now moving downstream very rapidly towards Gatton, where we are predicting a flood soon," he said. Mr Perkins said floodwaters that had already caused the Lockyer Creek, part of the Brisbane catchment, to rise rapidly would take about 18 hours to reach Brisbane.

Brisbane's last major flood, in January 1974, occurred during a La Nina event -- a Pacific climatic phenomenon often associated with high rainfall -- across Australia's eastern states.

Scientists have associated this year's unusually strong La Nina event with record rainfall and floods that have caused billions of dollars worth of damage in NSW and Queensland this summer.

La Nina's equal and opposite, El Nino, may be to blame for the recent decade of drought.

Mr Perkins said Brisbane had not experienced severe widespread flooding since 1996. "Then the river only got to 2.1 m -- this is another 0.9 m higher," he said. Mr Perkins said significant floods in the east Queensland town of Dalby were nearing a peak of 3.6m, at major flood level, and the Mary River at Gympie was expected to reach a peak of 19m last night. Bureau of Meteorology senior forecaster Ben Annells said southern Queensland could expect showers with a chance of thunderstorms for the next few days.