Lake Te Anau's slide towards its mimimum recommended (201.1 metre) equinoctial operating lake level has lake guardians concerned.

Guardian of Lakes Te Anau, Manapouri and Monowai (the Guardians) chairperson Madeleine Peacock said, "What we are concerned about is that obviously the lake levels are low and getting lower and there is no significant rain on the horizon to start topping the lakes up."

"Even though it is good for the shoreline to be exposed at times, it's not good for it to be exposed for a long period of time," she said.

Lake Te Anau. Photo: Southland App

Hydro energy generator Meridian Energy (Meridian) manages the level of Lake Te Anau by altering the flow of water through the Te Anau Lake control structure (TLC) and into the upper Waiau River. It also manages Lake Manapouri, and flows into the Waiau River.

Meridian is required to consult with both Ngai Tahu and Fish and Game, as well as receive sign-off to any changes from the Guardians. However, when there is a need to move outside its consented levels, final sign-off moves to Environment Southland (ES).

Under lake level guidelines, Lake Te Anau should not go below 201.1 metres during the (March - April) equnoctial period. Outside of this period the lake can operate between 201.1 metres and its extreme minimum, 200.86 metres, but only for 21 days at a time, or for just 42 days of the year.

Lake level guidelines are set from historical lake level data collected prior to the establishment of the TLC.

On Friday (25 March) Meridian gained the go-ahead to started reducing the flow from Lake Te Anau into the upper Waiau River from 115 cubic metres per second (cumecs) down to the river's minimum consented flow rate of 80 cumecs.

However, a week later (1 April) Meridian applied for and received conditional direction from ES to further reduce the flow into the upper Waiau River.

Upper Waiau River levels have been progressively dropping since January. Graphic: Environment Southland

Meridian Energy (Meridian) manager Andrew Feierabend said the flow into the upper Waiau River was now able to drop by 5 cumecs per day to a new minimum of 50 cumecs.

Flows and levels into the upper Waiau River have been dropping since January.

Peacock said, "We appreciate that Meridian are really stretched at the moment with having to manage these things. We know they are reducing their generation and continuing to reduce it, and that will help preserve the shoreline of Lake Manapouri, but reducing generation now cannot effect the flow of the [upper ]Waiau [river] or the level of Lake Te Anau."

"[the Guardians] will continue to work with Meridian and as things develop," Peacock said.

"If there is incursions, if there is any breaches of guidelines, then Meridian have internal processes that they have to follow and that includes reporting to the Minister of Conservation, the Minister for Energy, the Guardians. And they then have to report and give account for why these events happened."

"So there will be ongoing discussion around firstly managing the situation but afterwards being able to reflect on it, looking into this period, [and see if there] was there anything that could have been done differently to have managed it better," she said.