Brisbane is headed for its worst flood since 1893, with up to 40,000 properties at risk of being affected.

A disaster has been declared and an evacuation centre set up at the RNA Showgrounds at Bowen Hills.

Queensland Premier Anna Bligh tonight warned river levels would rise above the heights experienced in the catastrophic 1974 floods.

Ms Bligh said the situation developing in Brisbane and Ipswich to the west was “frightening”, with evacuations under way ahead of the Brisbane River’s expected peak on Thursday.