Japan's Mitsui & Co has made a $481-million offer to buy Australia's AWE, trumping bids from two other companies that are vying for a stake in a promising onshore gas field.

The Mitsui offer of A$0.95 a share in cash, or about A$594-million, marks the first full takeover bid by the Japanese trading giant for an Australian company and would make it the operator of a gas field for the first time.

Its bid is 14% above a cash and shares offer from Australian miner Mineral Resources already recommended by AWE, and 74% above AWE's share price last November when it announced a bid from state-owned China Energy Reserve and Chemicals Group (CERCG).

However investors bet on yet a higher offer emerging, sending AWE's shares up 18% to trade at A$1.00 a share.

"When you've still got two other parties that are active in the pursuit of this company ... anyone would be cautioned to call something a knockout bid in this situation," Royal Bank of Canada analyst Ben Wilson said.

RBC has an unrisked valuation of A$1.10 a share on AWE.

AWE said it was weighing the Mitsui proposal. Mineral Resources will have three days to decide its next move after AWE issues its response.

All three bidders have said they see strong potential in the Waitsia gas prospect in Western Australia, operated and 50%-owned by AWE.

"AWE's interest in the Waitsia Gas Project has the potential to be an attractive source of future gas supply to a wide range of Western Australian gas customers," Mitsui said.

The Japanese company is eager to gain experience as operator of a gas field, a Mitsui spokeswoman in Tokyo said.

"Having knowledge of an operatorship in an Australian gas field would help us win other energy deals in the future in the country," she said.

Mineral Resources MD Chris Ellison was not immediately available to comment.

China's CERCG, which sent its A$0.73 a share offer to AWE's shareholders last week, declined to comment.

Mitsui's proposal is subject to AWE terminating its agreement with Mineral Resources by the end of February 2.

Mitsui has worked closely with AWE on other gas projects in Australia and New Zealand and is also a stakeholder in major liquefied natural gas projects, as well as coal and iron ore mines.

The Japanese firm had considered bidding with AWE for Origin Energy's 50% stake in the Waitsia field when it was put up for sale in 2016, industry sources said, so it knows the asset and has not sought to inspect AWE's books.

Beach Energy owns the other 50% of Waitsia.