The victims of Queensland's devastating 2011 floods are partly to blame for damage to their properties because they didn't do enough to protect them and are guilty of "contributory negligence", dam operators claim.

Details of Seqwater's defence of a class-action suit came to light during the first week of a trial in the New South Wales Supreme Court, where law firm Maurice Blackburn is representing about 6000 victims of the 2011 deluge.

Water was released Wivenhoe Dam during the 2011 flood. Credit:Dan Saffron

It is alleged the Wivenhoe Dam operator failed to follow its own manual and should have taken into account rainfall forecasts before making extraordinary water releases, which caused extensive flooding in Brisbane, Ipswich and the surrounding areas.

Lead clients Vince and Maria Rodriguez, whose Fairfield Gardens business had to close after it lost $100,000 worth of stock, should have known their store was leased in a shopping centre that was build on land that flooded in 1974, the court heard on Thursday.