But the fate of the long awaited project might be sealed by events far beyond the borders of the ACT, according to a report to be published on Friday by Infrastructure Australia.

High speed rail has been on and off the agenda for Canberra policy makers and its business community since the 1980s, with various studies, ideas and campaigns coming to nothing.

Now, Infrastructure Australia is warning that a failure to act soon by state and federal governments might make the project much more difficult and expensive to realise.

Rapid growth on the fringes of Melbourne and Sydney threaten to add billions of dollars to the cost of building a high-speed rail line between the two cities, Infrastructure Australia warns.

IA says the governments of NSW and Victoria should get in early and buy land along the proposed rail corridor now, at an estimated cost of $720 million, or pay more than $3.5 billion later.

The agency says a failure to protect the corridor within the next three to five years could do serious harm to a project that will become vital given the populations of Melbourne and Sydney are projected to double by 2060.

In its report Corridor Protection, Infrastructure Australia says very fast trains could be running between Sydney and Canberra within 15 years and to Melbourne five years later.

Those indicative dates are based on work beginning on the main Sydney-Melbourne route by 2024, with a Canberra link to open by 2032.

But the job will be made much more difficult if action is not taken soon to protect the proposed route.

IA has identified about 60 kilometres of land on the urban fringes of Australia's two biggest cities that has been earmarked by planners for a future high-speed rail connection, but which is at risk of being gobbled up by developers instead.

Infrastructure Australia chairman Mark Birrell said New South Wales, Victoria and the Commonwealth had to commit now to acquire the land on which a high-speed rail line for Australia's east coast will one day be built.

Otherwise Australia could blow its chance to build a link for the benefit of future generations.

"It requires governments over the next three to five years to put aside funds which will reap a long-term return," Mr Birrell said.

"If we get this right we will avoid huge cost, disruption and engineering problems."

Infrastructure Australia has valued the at-risk land along the corridor at about $720 million, using data from the NSW and Victorian valuers-general.

It estimates that the land would cost at least $3.5 billion to acquire if it is sold to developers.

In its discussion paper to be published on Friday, the authority argues population growth in Melbourne and Sydney is rapid enough to justify opening the link as early as 2032, initially between Sydney and Canberra.

Mr Birrell said the project's long-term development phase brought a continual risk of having it pushed to the bottom of government priority lists.

"No one is saying we won't need high speed rail in 20 years but to do that you need to preserve the corridor now," he said.

"Otherwise there will be unnecessary and expensive tunnelling, compulsory acquisitions and the likelihood of community distrust."