Australia's top energy retailer Origin Energy dismissed talk a pair of investors led by Canada's Brookfield might change a A$15.5 billion ($11 billion) buyout proposal as energy market turmoil dragged down its half-year profit.

Origin CEO Frank Calabria made clear that what would be Australia's biggest private equity-backed takeover remains in play, after delays and new government policies sparked speculation the bidders might walk away or pare their indicative offer.

Since the group involving Brookfield and U.S.-based MidOcean Energy announced its approach in November, Australia's government has introduced coal and gas price caps and proposed gas export restrictions.

Origin meanwhile upped its earnings guidance for a second time in a month even after soaring coal prices pulled its underlying profit down 84% in the six months to December.

"The only proposal from the consortium is the one that was submitted late last year," Calabria told Reuters in a phone interview. "I wouldn't be speculating anything beyond that, at this particular point in time."

A spokesperson for the consortium declined to comment.

Calabria said the protracted talks were to be expected, even after the deadline for exclusive due diligence passed nearly a month ago.

"A transaction of this scale involving a consortium that are coming together to buy two different parts of the business, they have complexity and therefore it does take time," he said.

"The discussions are ongoing, they're constructive, the parties are actively engaged."

Last month, Origin raised its energy markets guidance to pre-tax profit of A$600 million to A$730 million for the year to June. On Feb. 16, the country's No. 2 power producer said it now expects pre-tax profit at the upper end of that range.

Still, Origin shares fell as much as 3% to A$6.92 by midafternoon, well below the North Americans' A$9 indicative offer, as investors fretted that wider energy market instability may sink the deal.

"The bidders are still very much interested in a deal, but it's going to be at a price significantly below the tabled A$9-a-share offer, given the energy markets uncertainty domestically and tumbling share price," said IG Australia analyst Tony Sycamore.