Mercury NZ chair Joan Withers will step down in September after 10 years as a director.

Withers had some time ago signalled her intention to leave the company this year and no let-up is expected during her final six months with the company, chief executive Fraser Whineray told executives and officials at a Mercury function in Wellington last night.

Withers, who also chairs Warehouse Group and is a director of ANZ New Zealand, joined the board of then Mighty River Power in August 2009 when the company was wholly state-owned. She took over as chair two months later.

During the following decade, the company commissioned the world's largest single-shaft geothermal turbine at its Nga Awa Purua site in 2010, consented two wind sites in the lower North Island, and commissioned the Ngatamariki geothermal plant in 2013. The same year it was the first of the SOE generators to list under the previous government's mixed-ownership model.

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In 2015 the firm shut its gas-fired Southdown plant to focus on renewables and in 2016 it adopted the Mercury brand. Last year it bought into Tilt Renewables to gain exposure to wind and solar in Australia and last month announced a $256 million wind development at Turitea.

Whineray said Withers is one of the country's hardest working directors and had made a tremendous contribution to the business, the rejuvenation of its board and its heightened focus on renewables.

"Joan just hasn't led, or talked, she has walked the walk and driven it."

Earlier, Withers told guests what a privilege it was working with a company whose heritage on the Waikato River stretches back almost a century, and which now has almost 20 years' experience in geothermal.

She cited the firm's Turitea investment, and the $250 million it has spent to date its on-going refurbishment hydro programme, as an expression of the confidence the company has in the New Zealand electricity sector.

"Everyone here, I think, really understands the importance of those sources of energy to a healthier, low-carbon future for our communities and our country," she said.

Confidence in the country's socio-economic and regulatory and political environment is also of "critical importance" in making that scale of investment.

She commended the "very constructive" approach taken by the government's electricity price review panel and said she hoped it would take the opportunity to improve the sector and build on its reputation as a global benchmark for a well-functioning system that delivers secure, environmentally sustainable and equitable power supplies.

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- BusinessDesk