TrustPower, the utility controlled by Infratil, boosted underlying annual profit 16 per cent, achieving wider margins while shedding customers in a tight retail market.

Underlying earnings, which strip out one-off impairment charges and changes in the fair value of assets, rose to $135.5 million in the 12 months ended March 31, from $116.5 million a year earlier, the company said.

Net profit rose to $131.7 million, or 41.8c per share, from $112.4 million, or 35.6c per share.

Total revenue rose 5.4 per cent to $807.1 million, with a 5.6 per cent gain in electricity revenue to $763.6 million. That was even as customer numbers fell to 209,000 as at the end of March from 221,000 a year earlier.

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"TrustPower continues to experience lower levels of customer churn than the market," the company said. "TrustPower refrained from actively acquiring customers to ensure a seamless transition for the replacement of its core customer billing and information system."

The company sold 3960 gigawatt hours (GWh) of electricity in New Zealand, down from 4033 GWh a year earlier. It boosted generation production 13 per cent to 2582 GWh.

"TrustPower expects electricity prices will become more volatile over time and, consequently, generation capacity able to meet peak demand will become more valuable."

With many of its hydro generation assets near peak capacity, it has embarked on a review to "identify potential enhancement opportunities that can increase peaking capacity in the short to medium term."

Last week, TrustPower finalised conditional agreements to expand its 100 megawatt Snowtown wind farm in South Australia, and entered a long-term supply arrangement with Contact Energy's majority shareholder, Origin Energy of Australia.

TrustPower declared a final dividend of 20c per share, taking the annual payout to 40c, up from an annual 38c in 2011. Shares closed up 2c yesterday at $7.72.