The country's biggest retail power company Genesis Energy has increased its stake in the Kupe oil and gas field.
Genesis has bought the 15 percent stake in the field from New Zealand Oil and Gas for $168 million.
This gives Genesis 46 percent of the field, with Australia's Origin Energy holding 51 percent and Japan's Mitsui the balance.
The offshore Kupe field produces gas, LPG, and condensate oil and Genesis said it fitted its plan to broaden its energy offering.
"This acquisition builds on Genesis Energy's strategy to create value by linking upstream fuel supply with electricity generation and consumer energy needs," said chief executive Marc England.
He said the field had strong cash flows to support its investment in new technology, and help to sustain high dividends to shareholders.
Genesis said it expected its operating earnings to be boosted, with a partial contribution this financial year and by about $15m next year, with a $2m increase to the bottom line.
The acquisition is being funded by existing debt facilities.
Meanwhile, NZ Oil and Gas said it had worked hard to find extra reserves in the field, and although the field has been its biggest earner the price offered by Genesis was too good to turn down.
"The transaction with Genesis allows New Zealand Oil and Gas to realise this upside now, while removing the risks of development and uncertainty around future market conditions," acting-chief executive Andrew Jefferies said.
He said the company would look to return $100m to shareholders once the deal had been completed, and it would still have about $100m for future acquisitions.
NZ Oil and Gas's share price has surged 28 percent this morning to its highest level since March 2015, while Genesis rose just 1 percent.