The 2007 Australian of the Year last night poured scorn on the idea that there was extra water in the Murray-Darling system that could be used to increase Melbourne's supply.

"Why would you take the water from an already stressed river system, and then they say 'Well, we're actually making more water in the end', which is bullshit," Professor Flannery said when answering a question during an address at RMIT University.

"Sorry, there is only a certain amount of water in the system.

"Why would you do that rather than trying to do something about coal-fired power plants, which consume 20pc of the water used in the state?"

Launching his contribution to the Quarterly Essay series, Professor Flannery said replacing Victoria's coal-fired electricity with cleaner forms of energy would ultimately yield a water bonus.

"To generate a megawatt of electricity you use about two tonnes of water in Victoria, so if you want to save water, switch off the light," he said.

Professor Flannery's comments came as Coalition leader Ted Baillieu yesterday backed his water spokeswoman, Louise Asher, despite Ms Asher being forced to correct her pledge that a Coalition government would not take water from the pipeline.

After five days of silence on the policy, Mr Baillieu acknowledged that water could flow from the pipe in a "transition phase" until Liberal-National plans for Melbourne's supply were complete.

He said pipeline water could also be used for critical human needs if there was a water crisis.

Ms Asher vowed late last week: "Our (Coalition) policy position going to the next election will be that we will not take water from the pipe."

One Liberal MP called the policy U-turn an "embarrassing stuff-up".

"We had our pants pulled down in public," the MP said.

Meanwhile, pressure on the State Government to adopt recycled water for drinking continues to grow, with a UN-sanctioned research group warning that Victoria's desalination plant could be one of the most energy-hungry in the world.

The UNESCO Centre for Membrane Science and Technology says Victoria's cold seawater would force the Wonthaggi plant to work harder - and use more energy - than most other plants.