In a new approach, Water Minister Lisa Neville has confirmed the $4 billion plant will now be switched on every year and fulfil a minimum water order to top up the state's dams as Victoria's population continues to swell.

With the first water from the trouble-plagued plant now flowing into the Cardinia Reservoir, Ms Neville said a minimum annual water order would mean better water security for Melbourne and better plant management, as well as helping stop large fluctuations in household water bills that can occur during dry years requiring big water orders.

"We do need to move past this concept that the desal [plant] is for a crisis only," she said. "It is there to avoid a crisis, so a minimum water order makes sure we can do that."

Despite some solid rain last winter and spring, the government has for the first time ordered 50 billion litres from the plant for the current year, which must be completed by June 30.

Claiming a win for Melbourne households, Ms Neville said water customers would not face higher bills to pay for this year's order. Nor will customers pay more over the next three years for the minimum water order, which is likely to be about 15 billion litres a year.

As a result, an average household would save about $12 on their bills this year, she said. Ms Neville said the saving had been made possible with "efficiencies" found within the water contract, although she did not provide any details about how this would work or where the money has been found.

The new approach to the desal plant, which is likely to fuel claims that the government is unfairly imposing it on households to justify its existence, follows news that the plant has been plagued by technical problems that have placed in doubt its ability to deliver the water on time.

As reported by Fairfax Media, ratings agency Standard & Poors warned of "serious financial consequences" for the plant if it failed to deliver the first 50 billion-litre water order by June 30.

Ms Neville said the minimum water order approach would reduce the risks of further technical problems, allowing the plant to operate annually rather than only sporadically.

"I am reminded of the Yes, Prime Minister episode where the hospital operates better without patients. We have this major asset that is unable to be tested year in, year out properly to be able to be maintained and assure Victorians that it is ready to operate," she said.

Commissioned during the so-called millennium drought, when Melbourne was facing the possibility of running out of water due to record low inflows, the plant has sat idle for years after solid rainfall from about 2007 onwards.

Nationals leader Peter Walsh slammed the change of approach for the plant, saying Ms Neville's claim that the permanent water order would not cost customers money was "absolutely ridiculous".

"It will cost them [water customers] money every single year, and if the minister is saying that can come from savings from the contract, those savings actually should be handed back to customers," Mr Walsh said. "It should not be Melbourne Water's customers' responsibility to help pay [plant operator] Aquasure to meet the contractual arrangements."

The first 50 billion-litre water order will take about three months to produce, meaning the deadline will be extremely tight.

Aquasure chief executive Matt Brassington said it was "more rewarding" for staff at the plant to actually produce water rather than merely maintain the plant.

He said the plant's electrical problems had now been solved, adding that the timing had been "appalling", with little time to deliver the maiden water order.

"We haven't got a lot time," he said. "It's nice to hear the noise now of the water coming in. We'll do every thing we can."

The state government's claim is that the plant is now crucial to Melbourne's water security. Ms Neville said the start of 2017 had been "extra dry", with Melbourne's storages about 66 per cent full, compared with 75.7 per cent at the same time two years earlier and 63.2 per cent last year.

Victoria recorded some good rainfall last winter and spring, but across Melbourne's four main catchments for January and February combined rainfall was 23 per cent below the 30-year average.

"The latest Bureau of Meteorology data shows Victoria is set for an 'Indian summer', with temperatures in March well above average and rainfall at a decade-long low," Ms Neville said. "Climate modelling also suggests there is an increased chance of an El Nino forming later this year."