Artist's impression of the City interchange for the proposed Canberra light rail.

Artist's impression of the City interchange for the proposed Canberra light rail. Photo: Supplied

The construction of the first stage of the Capital Metro light rail network will require all of the road surface on Northbourne Avenue to be dug up and replaced.

The project would also require the installation of new electricity substations every 2 kilometres along the 12-kilometre route from Civic to Hibberson Street in Gungahlin.

A report by consultants URS Australia prepared for the ACT government in April 2012 assessed options for the installation of light rail or rapid bus transport along the city to Gungahlin corridor. The government decided to proceed with the construction of a light rail system along Northbourne Avenue, onto Flemington Road to Hibberson Street.

The report said the construction of light rail infrastructure in the Northbourne Avenue median strip would require the entire road to be excavated.

''The alignment of this option uses a large volume of material on Northbourne Avenue to change the alignment, ie the entire existing road will be excavated and replaced,'' the report said. ''Also large volume truck movements are expected during construction. The alignment will require relocation of almost all services in the central median.''

The initial $614 million cost estimate for the project included $200 million for the relocation of utilities underneath Northbourne Avenue, such as water pipes or data cables.

Although the government eventually decided to run light rail down the Northbourne Avenue median strip, URS' preferred option was for kerbside light rail or rapid bus transit.

The URS report said this would ''act like a tram or streetcar with many stops and integration with verge-side pavements to help create an active cafe and retail strip along Northbourne Avenue''.

The report said it may be possible for trams to receive power from beneath the track, but this could be more expensive than traditional overhead power cables.

''Emerging technology exists to operate [the light rail] system with electric supply from beneath the track, which increases a city's attractiveness, but adds to capital costs.''

Power would be supplied from new substations at two-kilometre intervals along the corridor. Some of the substations could be located inside buildings.

''Generally, light rail projects were implemented in three to four years from the beginning of preliminary design to the end of testing,'' the report said.

Under its parliamentary agreement with Greens minister Shane Rattenbury, Labor has agreed that construction of the Capital Metro should begin before the 2016 election.

The government is considering whether or not to release key light rail planning documents.

Last month, government officials told an Assembly estimates committee that a submission on the project to Infrastructure Australia could be released upon application to Infrastructure Australia.

But Sustainable Development Minister Simon Corbell later wrote to the committee and said this was incorrect as the document was ''cabinet-in-confidence''.

Mr Corbell has undertaken to consider releasing the submission.