"We have been saying to shareholders, do not let this thing get too carried away because people will end up thinking that we are over a barrel, they will push the price too high and the deal will stall because we are economic rationalists," the Australian quoted him as saying.

While many analysts believe Origin's coal seam methane reserves in Queensland's Bowen and Surat basins are vital for BG's regional growth plans, Chapman said the UK gas major had sufficient global gas reserves to fulfil its supply contracts and also had access to enough local gas to seed a liquefied natural gas plant.

"The markets we have established in Asia Pacific, we can already supply them about three times over from our existing portfolio without one gram of LNG coming out of Australia," he added.

In a statement announcing the hostile $15.50 per share offer for Origin, BG sought to downplay the value of Origin's CSM reserves, questioning whether there had been sufficient drilling to justify the increase in the size of Origin's stated reserves since the parties began discussions.

"We are not saying the reserves will not be there, it is a prolific basin ... but like QGC (Queensland Gas Company) they are going to have to invest between $1 billion and $1.5 billion and spend three to four years proving up those reserves ... such that you are prepared to invest $8 billion in an LNG plant," Chapman added.

He admitted Origin shareholders were unlikely to make a decision before the emergence of more information about how much third parties were willing to pay for the company's CSM.

"Shareholders are saying we like the debate, we like to have the option to take either the cash offer … or the alternative, when it is defined," Chapman said.

Analysts expect broad proposals from interested parties in the next week before shortlisted candidates are granted access to Origin's books.