CONSTRUCTION work on thousands of Victorian homes has been approved by unregistered inspectors due to serious failings by the state's building regulator.

Documents show the Victorian Building Commission was repeatedly warned between 2003 and 2008 that several surveying firms were using unregistered inspectors. But it took four years to impose any disciplinary action against those involved.

Surveyors are required by law to use building inspectors registered with the commission to approve various stages of construction work, such as pre-slab preparations, steel structures and wooden frames.

Former planning minister Justin Madden was warned in 2007 and 2008 about the widespread use of unregistered inspectors - a practice that has potentially serious implications for insurance policy holders and the legality of house contracts of sale.

Commercial and construction law firm Lovegrove & Lord advised clients in 2007 that ''an insurer may be able to deny cover in circumstances where work has been carried out by an unregistered person simply because of the fact that legislation requires them to be registered''.

The commission is a statutory authority responsible for regulating construction standards and the permit system for works ranging from home extensions to high-rise towers.

The Age last week revealed 30 officials and consultants to the commission are being investigated for alleged corruption, misconduct and harassment.