The Airport Authority has said that the third runway will significantly expand flight capacity at the airport, which served 70.5 million passengers last year, and provide $58 billion in additional revenue by 2061. Supporters of the plan include Cathay Pacific Airways, Hong Kong’s flagship carrier.

“If there was already a third runway, it would be utilized,” said Will Horton, a senior analyst in Hong Kong with the Capa Center for Aviation, based in Australia. “Local airlines are not growing as fast as they could, while foreign airlines are waiting to add flights.”

But critics call the third runway project a white elephant, in part because it is unclear how much airspace mainland aviation authorities will allocate to Hong Kong in the coming decades, or whether the city airport can even hope to compete over the long term with its mainland Chinese counterparts.

Albert Cheng, an aircraft engineer and former Hong Kong legislator, said the city should not bother investing in a third runway because it had already been “marginalized by China in every aspect” and would eventually lose its cachet as Asia’s aviation hub, lowering demand for flights.

Another issue is whether the project’s proponents have accurately conveyed its environmental risks and whether the government should have allowed it to go forward.

Some critics say that the project should not have been approved because of the potential impact on dolphins and that HSBC, the project’s financial adviser, bears some responsibility because it has adopted the Equator Principles, a set of standards many institutions apply to ensure the projects they finance or advise do not harm people or the environment.