Mitsui's stake in the BassGas project, which includes the undeveloped Trefoil gas project, could be worth about A$360 million ($244 million), according to Credit Suisse. However, a person with knowledge of the asset estimated it at about A$140 million.

Mitsui Australia's spokesman said Rothschild has been engaged to advise on the sale of the stake, which Mitsui acquired with its takeover of oil and gas producer AWE last year. The planned sale was first reported by The Australian Financial Review newspaper.

BassGas, operated by Beach Energy, has been producing gas and liquids from the Yolla field in the Bass Basin off the state of Victoria since 2006. The gas is processed at a plant onshore and supplies the Australian east coast market.

Mitsui's share of output from the project in the year ended June 2019 was about 1.1 million barrels of oil equivalent, as calculated by Reuters based on Mitsui's 35% stake in Yolla and Beach's 53.8% share at 1.7 mmboe.

Beach declined to comment on whether the company might be interested in the stake or would have pre-emptive rights on it, as that is confidential.

The other partner in the BassGas project is Prize Petroleum, a unit of India's Hindustan Petroleum Corp, which bought an 11.25% stake in BassGas and a 9.75% stake in Trefoil for A$85 million in 2014.

Prize Petroleum declined to comment.

Mitsui's move comes at the same time that Exxon Mobil Corp has put its much bigger, but ageing Bass Strait oil and gas assets up for sale, which analysts and bankers have said would be of interest to Mitsui.

Mitsui declined to comment on whether it would be interested in the Exxon assets.

(Reporting by Sonali Paul, Editing by Shri Navaratnam and Sherry Jacob-Phillips)

By Sonali Paul