Emeritus Professor Sir Alan Mark, of Dunedin, took issue with commentary from Meridian Energy, which suggested a short period of low lake levels at Lake Manapouri would be acceptable.
When lake levels were lowered in 1972 several beaches were lost forever, Sir Alan said.
"Dropping the lakes below their natural minimum for just short periods, once every so often, can be highly damaging; once you’ve lost the gravel on the beaches over the edge of the shelf, it’s gone for good," he said.
Sir Alan was the first chairman of the Guardians of Lakes Manapouri, Monowai and Te Anau, a role he held for 26 years after the group was established in 1973.
The present-day Guardians recently knocked back a Meridian proposal to lower the lake’s minimum level for security of supply during a power crisis.
However, after Meridian and the Guardians met in Invercargill last week, Meridian Energy statutory advocacy and compliance strategy manager Andrew Feierabend said a compliance buffer of 20cm below the extreme minimum for Lake Te Anau could be used safely if it was infrequent for short periods such as five days, and cautiously, such as once in a decade.
Sir Alan disagreed and said lakes needed to be operated within their natural ranges.
Guardians chairman Darryl Sycamore said the Guardians and Meridian were asked by Minister of Energy Simon Watts to consider options to allow for additional electricity generation during low in-flow periods.
Several "nuanced" changes to the guidelines were supported by the Guardians, but those that allowed further drawdown of the lakes below the absolute minimum were rejected.
The group did not support three propositions that enabled reducing the absolute minimum permitted lake level, Mr Sycamore said.
"The concerns raised by Sir Alan relate to the lowering of the minimum lake level which then exposes the lakebed to the shoreline slumping risk during high-wind events.
"The social, environmental, and recreational impacts of reducing the absolute minimum lake level are all entwined and has informed our decision-making."
The Guardians’ formal response to the minister’s request to look into options for additional generation would set out the Guardians’ concerns with any reduction in the minimum lake level and the risk to shoreline processes, he said.
A Meridian spokesman said the power company appreciated Sir Alan’s "interest and his deep knowledge" of the lakes.
The potential impacts of any changes to lake levels were discussed at the recent meeting with the Guardians and the two groups were working on a response to the minister, who would "ultimately make the final decision", the spokesman said.
"The Government has been very clear in its desire to ensure there’s enough electricity for all New Zealand homes and businesses this winter, and Manapōuri Power Station has an important role to play in that.
"Meridian believes access to additional hydro storage is essential for security of supply.
"While around 60% of New Zealand’s electricity comes from hydro generation, only 23% of that capacity can be stored in lakes and Meridian’s own lake storage equates to only 15 weeks of average generation."
Mr Watts said he was expecting to formally receive advice later this month.
"It is pleasing see progress being made by The Guardians of Lake Manpouri and Meridian energy to investigate and advise if any extra generation can be provided at Manapouri while meeting environmental considerations.
"We have been liaising with both parties as their negotiations have been underway and expect to formally receive their advice in mid-March.
"The Conservation Minister, and I will then consider the advice before making any decisions."
