The water authority is offering vegetable growers at Werribee and Bacchus Marsh 500 megalitres of water from the Thomson Dam put in reserve for the MID.

It follows a decision by Southern Rural Water in November last year to send 2000 megalitres of water to the west.

But MID's customer consultative committee chairman Graeme Anderson said farmers were not consulted about this latest water allocation and were disappointed the board had broken their trust.

Southern Rural Water's water supply general manager Clinton Rodda has defended the board's actions, arguing the board valued the committee and was continuing to meet with it to discuss the matter.

He also argued the 500 megalitre emergency water for Bacchus Marsh and Werribee was just 0.2 per cent of the annual allocation in the MID.

Mr Rodda said the transfer would provide an equivalent of an additional eight per cent allocation for the Bacchus Marsh Irrigation District.

"Southern Rural Water needed to give Bacchus Marsh farmers certainty about the minimum amount of water that we could provide next season (an equivalent amount of water to a 25 per cent allocation), or many of them would go out of business,'' he said.

"There is urgency to do this now so farmers can plan their season to maintain long term contracts and business viability.

"Bacchus Marsh is only continuing due to the emergency water supplies provided from the Thomson Drought Reserve.

"This has been provided with the generous support of the Macalister customer consultative committee and Macalister irrigators.''

Mr Rodda said the transfer will only take place if inflows from this season are not sufficient to provide the equivalent of 25 per cent of water entitlements.

"The board has always valued the enormous contribution that the MCCC make on a range of issues that relate to Macalister irrigators,'' he said. "We are continuing to meet with the MCCC on the matter.''