OG Oil & Gas, the oil and gas arm of the Ofer Global Group, has formalised its partial takeover offer for New Zealand Oil & Gas after the New Zealand energy explorer rejected an "inadequate" partial offer from ASX-listed Zeta Resources.

OGOG is offering 77 cents a share for a maximum 70 per cent of NZOG, above Zeta's 72 cent offer but still below the 78 cent-to-93 cent range in a valuation NZOG's independent directors commissioned from Northington Partners ahead of rejecting Zeta's proposal.

Alastair McGregor, chief executive of OG Oil & Gas (Singapore), said his company doesn't agree with Zeta's assessment that NZOG is "essentially a cashbox" that is at a "strategic crossroad".

While Zeta wanted NZOG to return $50m to shareholders while slashing costs, OGOG sees NZOG as being "in an enviable position, with the right leadership and sufficient capital to take advantage of this attractive point in the exploration and development cycle."

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"We are faced with two starkly divergent opinions on NZOG's future," McGregor said in his covering letter for the offer.

"Now is the right time in the exploration and development cycle to invest in the oil and gas sector. After many years of lean investment, we feel there is a unique opportunity for companies that are willing and able to prudently deploy resources toward new exploration."

The Northington Partners report, released last week, said Zeta's offer "appears to take no account of exploration upside, which while risky, could be significant."

NZOG's target company report said the company's current strategy is to seek further investment in exploration and development opportunities over the next 12-to-18 months, which it believes is a superior plan to returning cash to shareholders and wouldn't be viable if the capital return was made.

OGOG currently holds 6.8 million shares or about 4.3 per cent.

Parent Ofer Global is chaired by Eyal Ofer and has a portfolio of international investments including shipping, real estate and banking.

NZOG shares last traded at 72.5 cents, valuing the company at $115.7m, and have gained 15 per cent this year.

NZOG has 159.5 million ordinary shares along with 8.3 million partly paid shares which aren't listed on the NZX main board.