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NEWS

View of Maraetai II from above
Maraetai II is one of the nine hydro stations on the Waikato River. Recent rainfall has helped increase the hydro system's capacity to generate electricity this winter.

Heavy rainfall, hydro water storage efforts combine to ease winter power pressure

Five major weather events over the past two months delivering 291mm of rain over Lake Taupō have eased pressure on the Waikato Hydro System, improving its capacity to generate electricity this winter.

Rainfall figures show 65mm fell on 9 May, and 53mm from 27-29 May. On 4 June another 62mm of rain fell and a further 61mm between 10-13 June.

The most recent rainstorm which covered most of the country delivered 50mm over Taupō from 27-28 June.

Lake Taupō is the major source of energy for the hydro system, which supplies about 10% of the country’s electricity supply.

The recent rainfall moves the lake level to 41cm above the average for this time of the year and 66cm higher than the same time last year.

Tracking towards the end of June, the lake storage was 138% of average. It compares to 1 June 2024 to 30 May 2025, when inflows into Lake Taupō were of the 4th percentile (4th lowest in 100 years).

Water levels from tributaries feeding into the Waikato River have also increased, helping to improve hydro storage.

The improved lake level is another important step towards helping maintain the country’s security of supply during the colder, higher-demand months has improved, and it reduces concerns of a repeat of last year’s energy shortfall.

Mercury’s Executive GM of Wholesale Markets, Tim Thompson, said prior to the big rainfalls, Mercury had already taken significant steps to conserve storage, heading into winter.

“We looked very carefully at managing the risk of another dry winter and our team worked hard to conserve hydro storage to ensure we were well placed to continue powering New Zealand over winter.

“When the rain arrived, we were in a good position to let Lake Taupō fill quickly to improve our hydro storage.”

Mr Thompson also acknowledged that strong thermal and gas availability from other generators had also helped improve security of supply this winter.

The Waikato Hydro System comprises nine hydro systems of the Waikato River. It is the main hydroelectricity contributor on the North Island, generating an average of 4140 GWh per year.

“Our hydro system is a winter-fed catchment, that means we rely on rainfall between May and September to resupply Lake Taupō.

“From January to March we reduce generation when there is lower rainfall to take pressure off the lake.