New Zealand Oil & Gas, the energy exploration company, has upgraded the developed reserves at the Kupe field off the Taranaki coast and is still looking at ways to expand untapped reserves in the project.

The gas and light oil field's developed reserve was lifted to 6.02 million barrels of oil from 5.22 million barrels and NZOG estimates there's likely to be an extra 3.6 petajoules of gas and 16.69 kilotonnes of LPG, the company said in a statement. The joint venture partners in Kupe are also looking at ways to develop undeveloped reserves but haven't made a final decision on whether to proceed.

"This reserve increase provides additional volume from within the existing development," chief executive Andrew Knight said. "It also provides further security that contracted volumes can be met without needing significant additional capital."

Wellington-based NZOG has a 15 percent interest in Kupe, sitting alongside Origin Energy with 50 percent, Genesis Energy with 31 percent and Mitsui at 4 percent.

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NZOG's share of the undeveloped reserve would be 3.2 million barrels if the investment went ahead, it said.

The company has responded to a slump in global oil prices by reducing exploration spending, quitting its secondary ASX listing, and lowering directors' fees by not replacing former chair Peter Griffiths.

The shares last traded at 48.5 cents, and have gained 14 percent this year.