Oil Search, whose assets are mostly based in the Pacific island nation, said it was reviewing the bid that will see Woodside offer one share for every four Oil Search shares.
Oil Search was valued at about Aus dollars 11.6 billion, a 14 percent premium to its closing share price of Aus dollars 6.73 yesterday when Woodside closed at Aus dollars 30.58.
The proposal is subject to satisfactory due diligence and regulatory approvals, as well as support from Oil Search's major shareholder, the PNG government.
The announcement buoyed energy stocks on the benchmark S&P/ASX200 index, with Santos closing 5.25 per cent higher and AGL rising 3.68 per cent.
More From This Section
Oil Search stressed that shareholders were "entitled to an offer which adequately reflects this value potential", pointing to its share in the massive USD 19 billion PNG liquefied natural gas project - the largest development ever undertaken in the Pacific country.
The company said along with its other low-cost producing assets it was well-placed to capitalise from a recovery in the oil price, which has roughly halved in a year.
Oil Search said it recorded a net profit of USD 227.5 million for the same period, a 49 per cent increase from the prior year, as higher sales volumes partly offset weaker oil and gas prices.