The O'Farrell Government has costed its six kilometre, inner-west Sydney light rail expansion at $176 million which works out at $29 million per kilometre, less than half the ACT Government's estimate of $66 million per kilometre.

The ARA says Canberra's $600-800 million estimate price tag for 13km of light rail is overpriced.

The nation's peak rail industry group, the ARA has been asking since last month for more details to find out why the costs in Canberra are so high.

ARA chief executive Bryan Nye says aside from the obvious price difference, the inner-west Sydney Light Rail expansion needs to navigate a much more complex terrain than the ACT Civic – Gungahlin route.

"Despite having to navigate through a significantly more congested urban environment, the Sydney Light Rail expansion will cost less than half that of the Canberra light rail proposal," he says.

"The ACT government's estimates would make Canberra's light rail line one of the most expensive lines in the world on a per kilometre basis.

"If a city like Sydney can cost light rail at $29 million, claiming it would cost $66 million in Canberra is outrageous. Something doesn't add up."

The estimated cost of the Sydney light rail expansion is in line with industry estimates, such as those provided by the Tourism and Transport Forum, placing the cost of light rail at $20-$40 million per kilometre.

"We have two light rail projects, one which has been costed in line with industry estimates, the other which exceeds industry estimates by $30-$40 million per kilometres," says Mr Nye.

"With such a discrepancy, I have grave doubts as to the accuracy of the ACT costs and urge the ACT Government to release its calculations in full."