Water Minister Tim Holding had promised the pipe's first 75 billion litres would arrive by December 31. But evidence given at a parliamentary inquiry yesterday makes that goal highly unlikely to be achieved.

While still mathematically possible, pumping plans outlined by Melbourne Water yesterday suggest a shortfall of close to 10 billion litres is likely by December 31.

With a state election due in late November, the pipe flows loom as a political time bomb for the Brumby Government, which is already fearing an electoral backlash over water issues.

Melbourne Water officials told yesterday's inquiry that about 2.5 billion litres had gone down the pipe in its first five weeks, despite its being capable of delivering almost 13 billion litres over that period.

Pipe project director Rod Clifford confirmed that mechanical failures and storms were to blame for shutdowns on the pipe in recent weeks.

Two of the pipe's 12 massive pumps have suffered outages, with one not expected to start working again until next week.

Mr Clifford said such issues were minor and common during the ''running-in'' period of major projects, but he added that teething period would stretch into April.

The government will only use the pipe in irrigation seasons, meaning it has fewer than 200 days remaining this year to get the water to Melbourne. It will need the pipe to operate at its maximum rate over those days - and avoid shutdowns - to meet its 2010 promise.

In a blow to those hopes, Mr Clifford told the inquiry the pipe would typically run at about 83 per cent of capacity.

Water Minister Tim Holding was still hopeful yesterday that 75 billion litres could flow by December 31, but he stressed that Melbourne would still get the water eventually, even if the timeline was missed.

Opposition water spokesman Peter Walsh said it was highly unlikely the 2010 promise could be kept.