Competition tsar Rod Sims has indicated he remains in two minds about approving what he has described as a "very complex" deal by Virgin Australia to wrest control of Tiger Australia.
Mr Sims said the deal stood in stark contrast to the Qantas and Jetstar request to work more closely with joint venture airlines in Asia, to which the regulator gave final clearance on Tuesday.
"[The Virgin deal] is a very complex matter, whereas Jetstar started out complex and got simple," he said.
On the eve of an expected green light for the Qantas-Emirates alliance, the competition regulator has allowed Qantas and its budget offshoot to co-ordinate with Jetstar's joint venture operations in Singapore, Vietnam, Japan and Hong Kong on passenger and cargo services for the next five years.
The regulator said the better co-ordination of services posed "little, if any, detriment" because the airlines faced stiff competition from other carriers on the mostly intra-Asian routes they flew.
With Qantas about to step up its focus on Asia, the regulatory clearance will allow it to better co-ordinate flights from Australia with connecting services in Asia by Jetstar affiliates. Qantas's alliance with Emirates is focused primarily on destinations in Europe via Dubai in the Middle East.
The expected final clearance on Wednesday for the Qantas-Emirates alliance - which is scheduled to begin on Sunday - leaves Virgin's bid to take a controlling stake in Tiger as the last big aviation deal still before the Australian Competition and Consumer Commission.
Mr Sims said he expected it to be another month before the watchdog made a decision on the Virgin deal because of the need for it to assess crucial information from Australia's second-largest airline.
The regulator had been due to hand down a decision on March 14 but postponed it after requesting more details from Virgin.
Mr Sims declined to respond to Virgin chief executive John Borghetti's recent threat to walk away if the regulator forced it to commit to growing Tiger's fleet.
Mr Borghetti has said Tiger's fleet could be tripled to 35 planes within five years but insists he cannot promise such growth because of the volatile nature of the industry.
"It is up to John Borghetti to put views that they want to put," Mr Sims said.
While the deal would help Tiger better compete against Jetstar, Mr Sims said he remained concerned it would leave Australia dominated by two airline groups.