Qantas Airways plans to highlight the customer benefits of an expanded alliance with China Eastern after the competition watchdog said it could lead to higher airfares.
The Australian Competition and Consumer Commission says it proposes to reject plans by Qantas and China Eastern to co-ordinate services between Australia and China.
The tie-up could result in "significant public detriment" by giving the airlines the ability and incentive to limit capacity or increase fares between Sydney and Shanghai, the ACCC says.
But Qantas International chief executive Gareth Evans has countered that more than 20 airlines already provided services between Australia and mainland China in a highly competitive market.
"New traffic rights recently granted to Chinese carriers means the competition in this market will only increase, which underlines the importance of Qantas forming a strategic partnership with China Eastern so that we can strengthen our network and scheduling offer to customers," Mr Evans said in a statement on Tuesday.
"Over the coming weeks, Qantas and China Eastern will work with the ACCC and highlight the customer benefits of the proposed partnership."
Qantas argues that the tie-up would facilitate commerce between the two countries and deliver better departure and arrival schedules, reduced transit times and a wider range of onward connections within China and Australia.
The airlines, which already have a codesharing agreement, announced plans last November for a new deal that could open up new routes between Australia and China.
The pair planned to create a five-year joint venture that would bring together operations at Shanghai International Airport, cutting transit times and widening the range of connections.
The new agreement required approval from regulators in both countries and the airlines had hoped to start the new joint venture in mid-2015.
But ACCC chairman Rod Sims said Qantas and China Eastern already together accounted for more than 80 per cent of capacity on direct services between Sydney and Shanghai.
"They are the two major airlines on the route and the only airlines offering daily flights, and so the major competitive constraint on each other," he said in a statement on Tuesday.
"Competition between them will be greatly reduced under the proposed agreement."
Mr Sims said there would be some limited benefits from the deal, including savings in processing passengers and moving freight.
However, the ACCC believed the range of travel options available to passengers for journeys beyond Shanghai would not automatically be increased.
The watchdog is seeking submissions from interested parties in relation to its draft determination.