The tide is out at Lake Taupō as a dry winter takes its toll.

While the ducks are enjoying feeding in the pools left behind by the receding water’s edge, the low lake levels and dry winter is causing headaches for hydro-electric power stations along the Waikato River, where 10% of the country’s electricity is generated.

Electricity generation company Mercury manages the flow of water out of the lake to feed its hydro-electric power stations, and is required to keep the level between 357.25 to 355.85 metres above sea level - a range of 1.4m.

By the end of July this year, the level had dropped to 356.13m - 45cm lower than usual for the time of year, and more than 1 metre lower than it was at the beginning of the year, thanks to a winter which is shaping up to be the driest since 1992.

Mercury executive GM, portfolio Phil Gibson said they had reduced electricity generation from the company’s nine power stations along the Waikato River to help address the lower lake levels and were calling on other renewable energy sources like wind and geothermal to help meet the shortfall.

“We have a team monitoring the lake levels very closely and at this stage we are not taking additional action beyond the careful management of the hydro catchment, as we always do. We are experienced in caring for the lake and river during low flow events and take it seriously.”

The water level of Lake Taupō during dry years, with the minimum level allowed indicated by the black line. Graph / Mercury

Exceptionally high lake levels have been causing erosion concerns for some lakeside communities over the past two summers, with the level actually going 1.5cm above the maximum allowed on February 1, 2023.

The lake was sitting 1.21m lower than that by August 14 this year although it has risen by 15cm in the past two weeks thanks to rainfall in the lake catchment.

Gibson said lower than normal winter rainfall had also lowered water storage for other hydro power plants in the South Island. Low wind at wind generation sites had also exacerbated the issue in July and August.

“This winter is shaping up to be the driest weather period for the hydro electricity sector in New Zealand since 1992.

“The important thing is that our customers don’t really care about rain, they want power and we need to supply it.

“Last year we spent most of the year battling too much in-flow [at Lake Taupō] and couldn’t keep it down. This year is the opposite.”

Mercury Energy's Phil Gibson. Photo / Mercury Energy

He said they tried to keep at least 20cm away from the upper limit because that was “just one big storm” away from going over the top of the range.

When they get within 10cm of the bottom of the range, they had to be conservative and once at the bottom of the range, which has never happened before, they had to hold it there, which could cause issues for the river downstream, and there was the potential to run out of water.

There are more than 30 streams and rivers flowing into Lake Taupō, the largest of which is the Tongariro River at the southern end of the lake, which is fed by the Kaimanawa Range and the eastern slopes of Mt Tongariro, Ngauruhoe and Ruapehu.

“Tongariro can get pretty skinny, pretty dry,” Gibson said.

He said they were using the recent rainfall to build storage and the latest long-term forecast suggested there could be more rain on the way.