The Guardians of Lakes Manapouri, Monowai, and Te Anau will not support a Meridian Energy proposal to be able to lower Lake Te Anau an additional 20cm beyond its absolute minimum in an extreme circumstance.
After a year of ongoing discussions, the Guardians have formally decided to not back one of two proposals to amend how Meridian can manage the hydrolakes.
The rejected proposal would have allowed Meridian Energy to lower Lake Te Anau for another 20cm beyond the absolute minimum level the company is currently allowed to access. The drawdown was intended to occur only when in-flows to the lake were at an extreme low.
Guardians chairman Darryl Sycamore said they collectively voted against the proposal because it could potentially affect the values driving the lake level guidelines, specifically requirements in place to protect the existing patterns of the lakes, their ecological stability and the recreational value of their shorelines.
"The Guardians also noted a lack of any extreme low lake level precedent in the period of natural record, which reinforced their conclusion any further lowering of Lake Te Anau was not appropriate."
The mandated lake level operating guidelines are in place to maintain the natural state of the lake shores while at the same time optimising energy production. Meridian Energy needs the consent from the Guardians to make such amendments to the guidelines.
The lake level guidelines are separate from the resource consents Meridian Energy has to operate the Manapouri Power Scheme.
Meridian Energy statutory and compliance strategy manager Andrew Feierabend said the 20cm proposal was intended to provide a security supply in the extreme case of lake levels going so low that they could seriously hinder energy production.
Climate change was also a factor influencing the proposal, since predictions show Southland becoming drier during the autumn and wetter during the summer, he said.
He said with the proposal came guidelines that Meridian would have to meet to show it had made its best effort to avoid lowering Lake Te Anau an additional 20cm, such as having to turn off the station at night to preserve lake levels.
Meridian Energy would now need time to take stock of the Guardians' rationale behind the decision before deciding on what next steps it would take, he said.
The second proposal, still on the discussion table, would allow Meridian Energy to increase the speed with which it could lower the lakes when they were at their low operating range.
Mr Sycamore said the Guardians remained open-minded to this part of the proposal, noting it could benefit both lakes' ecology.
"In our view, the proposal offers a genuine opportunity to consider all the information obtained since the inception of the power scheme and maybe reset the ecology to better mirror how the environment would perform naturally."