After four years of inactivity, Victoria’s desalination plant is churning.

Fifty gigalitres of desalinated water is flowing into the Cardinia Reservoir, and the Victorian Government says it is ordering another 15 gigalitres a year for the next three years.

Water Minister Lisa Neville said the costs of the upcoming orders would be offset by new efficiencies in the plant operator's contract.

“In that three-year period for either this 50 gigalitre or for that minimum water order over the next three years there will be no additional cost to people's water bills for that water order,” she said.

The plant has sat idle since it was completed in December 2012.

It made news earlier this month after an electrical fault forced the operator to install 30 shipping-container-sized diesel generators as a back-up power supply.

The state’s water storages are about two-thirds full, leading Opposition water spokesperson Peter Walsh to claim the order was not necessary.

“It's absolutely ridiculous to say that having a permanent desal order will not cost Melbourne Water customers money, it will cost them money every single year,” he said.

“If the Minister's saying that could come from savings from the contract, those savings actually should be handed back to customers.”

Ms Neville said he did not understand the purpose of the plant.

“We do need to move past this concept that the desal is for a crisis only. It's there to avoid a crisis, so a minimum order makes sure that we do that,” she said.

“It also stops a major fluctuation in water pricing for customers, so you [order] 50 gigs or 100 gigs and all of a sudden that one year you water price goes up substantially on your household bill. This stops that … because smaller orders are cheaper for customers.”