Two of Queensland’s major dams have lowered their storage capacity, and will remain lowered for over half a decade.

The owner of the dams, SEQ Water, claimed lowering the storage of Somerset and Wivenhoe dams would not affect the security of the region's water supply, but would instead allow for upgrades as part of a Dam Improvement Program.

Wivenhoe's capacity was lowered to 90 percent of its previous full supply volume, while Somerset was reduced to 80 per cent.