Genesis Energy has confirmed the Kupe gas and oil field, off the south Taranaki coast, has bigger reserves than previously thought.
The power company said it had reviewed the recent data provided by the field operator, Australian Origin Energy, and had confirmed the field's total reserves were up more than a third to almost 37 million barrels of oil equivalent, from 27 million.
Genesis owns 31 percent of Kupe, Origin Energy owns half, while New Zealand Oil & Gas owns 15 percent and Japan's Mitsui holds 4 percent.
Genesis chief executive Albert Brantley said the increase in reserves extended the commercial life of the field and would save it having to spend on future development for the time being.
"Kupe is a consistent contributor to the company's diverse income streams. This significant reserves upgrade provides long-term fuel supply and revenue upside."
Mr Brantley said Genesis was awaiting the results of a further study, expected to be completed in June, before determining options for future development of the field.
New Zealand Oil and Gas said Genesis's upgrade was broadly aligned with the upgrade it announced in late October.