Ms Gillard told a breakfast event in Canberra yesterday that the long-mooted high speed rail system linking Australia’s east coast capitals would cost a “lot of money” and that the population density did not exist at present to support the project.

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And Qantas chief executive Alan Joyce became the latest public figure to cast doubt on the feasibility of a fast rail link. Another airport for Sydney was the only solution to its air traffic problem, he told the National Press Club.

But Canberra Airport, the city's major proponent of fast rail and rapid train systems, insists the link is viable.

The NSW infrastructure authority was also dismissive of the idea last week, writing in its State Infrastructure Plan that the benefits of the system were not worth its estimated $86 billion price tag.

Canberra's very fast train advocates say a link could be built linking central Canberra to the Sydney CBD for about $10 billion.

But Ms Gillard was not optimistic about the broader system when asked by Canberra businessman Tim Efkarpidis if she supported a high speed rail link for the capital.

''We've had Minister [Anthony] Albanese scope all of this out, through a feasibility study and the truth is it's a lot of money for a fast train that will do not only the interconnection from Sydney to Canberra, but also the interconnection to Melbourne,'' she said.

''I think it's some time away, before you would see population density at a point where it would make it viable.''

Chief Minister Katy Gallagher was more supportive of the link, but indicated that she, too, saw the project as happening in the long term. ''It's no secret that we support the fast train to Canberra, but like the Prime Minister has outlined, it's not sitting on our budget books,'' she said. ''We'll continue to support the work that's being done and to work with the Commonwealth to complete the studies.''

Mr Joyce told the National Press Club yesterday there was ''no point building fast train links to places like Canberra and assuming that will fix the problem. I don't think even Michael O'Leary from Ryanair would have had a goal to claim that Canberra was a second Sydney airport, and he makes outrageous claims on airports around the world.''

But Canberra Airport chief executive Stephen Byron said an expanded role for the capital's airport was a ''game changer'' for rapid rail.