That warning came from Meridian's water infrastructure development manager Nick Eldred during an Environment Canterbury (Ecan) resource consents hearing in Oamaru this week.
Irrigators fear their traditional 100% reliability for existing and future irrigation schemes will be lost under the Waitaki catchment water allocation regional plan's minimum flow of 150 cumecs.
Some at present have minimum flows as low as 80 cumecs before they have to stop taking water.
They want to ensure that reliability continues by Meridian releasing extra water above the minimum flow from the Waitaki dam to cover what irrigators take out of the lower Waitaki River.
Under its plan to build the $900 million north bank tunnel concept (NBTC) power scheme between the Waitaki dam and Stonewall, Meridian has proposed making that extra water available.
However, if NBTC does not get resource consent approval or is not built, irrigators are negotiating with Meridian in an attempt to have the extra water made available.
Mr Eldred said Meridian was committed to those ongoing discussions and recognised it would be preferable to have such an arrangement.
To meet the irrigators' request, Meridian would have to retain water in its main storage lakes of Pukaki and Tekapo during the winter and spring.
That water would not be available to generate electricity.
Mr Eldred said that would affect the two lakes' minimum and emergency zones used by the Electricity Commission to instigate conservation measures during an electricity crisis in dry years.
Providing the extra water for irrigators would mean the minimum and emergency zones could be entered earlier.
Meridian did not believe it could, or should, enter into commitments . . . that impact on the security of electricity supply to the South Island and New Zealand, he said.
While a loss of reliability would be a cost for irrigators, they had the ability to manage dry years risks through on-farm water storage, hay grain or silage.
However, Meridian was willing to continue to negotiate with irrigators to reach an agreement beneficial to both parties, Mr Eldred said.
The hearings panel is considering 56 applications to ECan to use water from the Hakataramea catchment, lower Waitaki River and its tributaries.
Those applying for resource consents have presented their evidence between August 11 and 22 and again at the start of this week.
Meridian on Monday and yesterday presented evidence conditionally supporting the applications.
Yesterday afternoon, the panel started to hear evidence from individuals and groups opposed to the consents being granted.
That is scheduled to continue until next Thursday, with applicants having a right of reply and the hearings due to finish on September 12.
