Victoria’s Opposition has slammed the state’s Labor government for a $27 million water order.

The deal was struck with the Victorian Desalination Plant to cover supplies for the coming summer.

Shadow treasurer Michael O'Brien says the water is not needed.

He said the state’s dams were at over 60 per cent full, and rainfall is expected to continue at an above average rate.

“This is simply Daniel Andrews increasing people's water bills to prove that his giant desal plant, his giant white elephant, actually works,” Mr O'Brien told reporters.

The order marks the first time the controversial desalination plant will actually be used, despite being completed in 2012.

But Acting Water Minister Martin Foley says recent rainfall has only brought a marginal increase for reservoirs.

“The recent rain does not make up for the prolonged below average rainfall,” he said.

“Melbourne’s storages are at 60 per cent. This time last year they were at 68 per cent.”

The 50 gigalitres of desalinated water the Government ordered is due to be delivered in the summer.

When the order was made, Water Minister Lisa Neville said it was based on advice from Melbourne water retailers after two years of big declines in storage levels.