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Mr Barnes joined Contact from former parent company Origin Energy in April 2011.
During his tenure, the firm reworked its Wairakei geothermal field, including commissioning a bioreactor to reduce discharges into the Waikato River and constructing its new Te Mihi power station there.
At the same time, the firm optimised the Ahuroa gas storage facility and Stratford peakers, shut the Otahuhu B power station in Auckland and re-established itself as a stand-alone business when Origin sold out in 2015.
Throughout that period it also significantly restructured its retail activities, including testing bundled offers with broadband, and has repositioned its commercial and industrial business to help businesses reduce their emissions.
Mr Barnes says 2019 will be his last full year in the role.
"I know that the company is in a strong position, with excellent prospects, and with a talented and committed team in place.''
Chairman Rob McDonald says the firm has commenced a selection process with the support of international recruitment consultancy, Russell Reynolds.
"Although the development of internal candidates has been a priority over the last few years, the board will also consider external candidates. We expect to be able to provide an update in coming months.''
Mr McDonald noted that under Mr Barnes' leadership, shareholders had enjoyed compound annual returns of 15.8%.
He said the firm's $2billion investment in renewables, flexible thermal generation and enterprise-wide systems was matched with an improved safety culture, more engaged employees and customers advocating for it in greater numbers.
- GAVIN EVANS