Contact Energy is continuing to lose customers after pushing up power prices by as much as 12% last year.
At the end of March, the country's largest listed power firm had 487,000 electricity customers, a 2% drop compared to the end of 2008.
Amost 6% of Contact customers have moved to another power company over the past year.
Meanwhile, generation fell nearly 6.5% in the first three months of the year as Contact shut its gas fired Otahuhu power plant for maintenance during the period.
The company received an average $33.28 per megawatt-hour for its generation, compared to $107.47 per megawatt hour a year earlier.
Last year, generation prices rose due to drought and low hydro lake inflows.