Mercury Energy says third-party costs are behind a 10% increase in the retail price of gas.
About 39,000 customers, mostly in Auckland, will pay about $70 per year more from next month.
Mercury Energy has written to gas customers telling them of the increase which it says is its first in 18 months.
General manager James Munro says it is not due to any internal costs or profit on Mercury's part.
He says the increase is down to the company passing on increases in third party costs, which are beyond their control.
A spokesperson for Contact Energy says they have not made any decision on price changes, but that there was upward pressure on pricing throughout the industry.
Genesis Energy says it has no plans to charge more. It reviewed prices in July.