A report to be issued today says airlines may try and compete to stem the loss of passengers to high-speed train on the lucrative route between the two capital cities.

But this would leave the busy route with such a big cut to frequency it would be unviable and airline services would no longer exist.

This is the view of the Canberra Airport, which is issuing the report on the impact of a second Sydney airport not being built, which is based on the ''High Speed Rail Study phase one'' and ''Joint Study on Aviation in the Sydney Region''.

The airport's report says if high-speed rail went ahead, more passengers would travel between Sydney and Canberra, via Canberra airport, aboard fast trains.

They would arrive by train in Sydney's central business district at least an hour quicker than if they arrived at Kingsford Smith Airport, where they would run the gauntlet of increasing traffic congestion and strong winds, which already restrict flights.

Infrastructure NSW dismissed the high-speed rail scenario last week, saying operating costs would to be too high compared to air transport.

Canberra Airport nominates Badgery's Creek in Sydney's outer west as the best option for a second airport.

But the airport believes a second Sydney airport will never built there, nor at Wilton, south of Sydney, because of political opposition, a view shared by NSW Premier Barry O'Farrell.

According to Canberra Airport's forecasts, nearly 12 million passengers will be using high-speed rail between Canberra and Sydney CBD by 2036. The airport committed $140 million to building a new high-speed rail terminal earlier this year and has released a YouTube clip promoting the project:

Airport managing director Stephen Byron said the case was simple given Kingsford-Smith Airport would be at capacity by 2027.

The growing passenger numbers from Sydney's overflow would make high-speed rail a viable solution for aviation capacity needs.

''It is only 15 years until KSA is at capacity according to the Joint Study on Aviation Capacity for the Sydney Region and if nothing is done the revenue forgone for NSW and Australia will be billions of dollars,'' Mr Byron said.

The Canberra Airport study concludes that in 2030, the number of passengers overflowing from a capacity-constrained Sydney airport and using Canberra Airport and high-speed rail will be 2.5 million. This would boost the total number of high-speed rail passengers by 37 per cent to 9.3 million.

''With HSR from Canberra Airport, passengers will reach the Sydney CBD in 57 minutes - faster than from Wilton, Badgery's Creek, or even KSA given the ground transport forecasts,'' Mr Byron said.

''And although they show Canberra Airport's relative advantage, the raw travel times fail to capture the biggest transformation of the HSR option: the certainty of travel time.

''The HSR will be on time, every time, as it is elsewhere in the world with average delays measured in seconds, not minutes.''

Mr Byron said a high-speed rail link between Sydney and Canberra would cost about $11 billion, equivalent to building a new second airport for Sydney at Wilton.