Estimates of the reserves in the Kupe oil and gas field off the Taranaki coast have been increased by more than a third.
The field's developed reserves have been increased by nearly 35 percent and NZ Oil & Gas, which owns 15 percent of the field, said the new estimate was equivalent to about 5.6 million barrels of oil.
Estimates of reserves for the field's natural gas and LPG were also up, and more work was being done on estimating undeveloped reserves.
Given the Kupe field is NZ Oil & Gas's major asset, the lift in reserves was described as significant, and would allow it to fill its contracts without having to spend a lot more.
Other partners in the Kupe field include the power company, Genesis, with nearly a third, Australia's Origin Energy, which owns 50 percent and operates the field, as well as Japan's Mitsui, which has a small stake.