Victoria's dry winter is likely to trigger the largest order of desalinated water the state has seen, in a decision that could challenge the government's commitment to keep water bills down.

Melbourne’s water storage levels have virtually flatlined this winter, for the first time since the millennium drought broke.

They are at 60 per cent, their lowest level in late winter since 2010.

Melbourne Water has advised the government to next year order 100 gigalitres of desalinated water, almost one quarter of the city’s annual demand.

This would be followed by an even larger order of 125 gigalitres of desal water in 2020.