The Victorian Government expects to save 225 billion litres a year, later increasing to 425 billion litres, by reducing losses from the food-bowl irrigation system in the state's north.

But preliminary estimates for 2008-09 from Goulburn-Murray Water indicate that only 343 billion litres will be lost this year — a record low reflecting impoverished flows through the leaking irrigation channels.

Critics said the continual decline in water losses raised serious questions about whether enough could be saved for the Government to pipe 75 billion litres annually to Melbourne.

Only a fraction of water lost to seepage and evaporation each year is expected to be recovered under the Government's Foodbowl Modernisation Project.

The first-stage plan to save 225 billion litres and share it among the farmers, rivers and Melbourne assumed losses would continue to average about 900 billion litres a year, as they had for the past century.

Losses have been less during the past decade's drought, averaging 690 billion litres.

Just six months ago, Premier John Brumby predicted losses would be about 500 billion litres this year.

Ken Pattison, a member of the Plug the Pipe campaign opposing the pipeline and a former director of Goulburn-Murray Water, said it was a "physical impossibility" to make the savings the Government had promised.

"The assumptions they based the project on are proving to be fundamentally flawed," he said.

"It is always going to be mostly an open channel irrigation system, and there will be these fixed losses where nothing can be done to save them."

The manager for Environment Victoria's healthy rivers campaigns, Juliet Le Feuvre, said it is unlikely the Government could fulfil its promise for Melbourne's supply if worst-case climate change predictions were to be fulfilled.