The completion date for the troubled Ararat Prison redevelopment is again in doubt, with the private consortium responsible yet to sign a builder.

The $400 million project is already two years behind schedule after the initial builder went broke and the original consortium set up by the previous Labor government collapsed.

The Baillieu Government helped form a new consortium and hoped that major work would begin early this month.

But a spokeswoman for the likely builder, Brookfield Multiplex, says it has not yet signed a contract.

Regardless, the Government is confident the prison will be finished as promised, by the end of next year.