Head of service Kathy Leigh's decision to refer the leaking of the draft Capital Metro rapid business case to ACT chief police officer Rudi Lammers sparked an investigation by the criminal investigations unit, The Canberra Times can reveal.

The source of the document's release, a criminal offence and breach of public service guidelines, was not found and the investigation was eventually closed.

Leaked to The Canberra Times in May, the business case revealed that bureaucrats from the Capital Metro Agency suggested the project's first stage be built only as far as Exhibition Park and a possible early extension be added to provide trams to the Russell precinct and the Parliamentary Triangle.

ACT Policing would not comment on the scope of the investigation or how many public servants had been interviewed, citing privacy concerns and protection of "police methodology".

Police did not seek access to a copy of the document in question.

Ms Leigh referred the matter to ACT Policing after Attorney-General Simon Corbell gave her responsibility for dealing with the leak on May 27.

"ACT Policing Criminal Investigations subsequently conducted an investigation," she said.

"Unfortunately, the investigation did not result in identifying the individual(s) that carried out the unauthorised disclosure of the document.

"This was not announced at the time on ACT Policing's advice so as to not interfere with the investigation process."

The document showed that the light rail project's focus on the Northbourne Avenue corridor could suck development and demand from other parts of Canberra and recommended that the government place a higher priority on the corridor, calling for decisions "that deliberately favour it over other precincts across the ACT".

It also referred to the planned redevelopment of public housing for more than 470 tenants along Northbourne Avenue and raised questions about patronage levels needed to make the tram line viable.

The rapid business case called for nine development zones to be created in the area, and for an increase in building height restrictions within 500 metres of the future tram line.

In a statement, Ms Leigh said the matter represented a breach of cabinet confidentiality. She received a verbal briefing from police after they concluded the investigation.

Ms Leigh said cabinet security was taken seriously across the ACT government and confidential papers were given only to officials who genuinely needed to know their content.

"Cabinet processes and procedures are continually being revised to ensure the security and handling of cabinet-in-confidence information. And as a result of the investigation into the unauthorised disclosure of the rapid business case document, all directorates through directors-general have been reminded of their obligations around the security and handling of cabinet papers."

Section 153 of the Crimes Act makes it a criminal offence for an official to disclose, without lawful authority, any information which comes into their possession as an officer of the ACT.

Officials are also under a duty not to disclose any information received during their employment under section 9 of the Public Sector Management Act.

Ms Leigh highlighted measures across the ACT public service designed to remind employees of their obligations when they came into contact with cabinet-in-confidence information.

She said the ACT Government Cabinet Handbook, staff training and reminders from managers made it clear leaking of sensitive information was not acceptable.

An ACT Policing spokeswoman would not say how long the investigation took but she said police would not make any further public comment.

The final business case for the tram line from the city to Gungahlin will be released on October 31.

Mr Corbell, who also serves as Capital Metro Minister, said the peak-hour commute from Gungahlin to the city would take more than 50 minutes by 2031 if light rail was not built.

The 12-kilometre line is expected to be operational by 2019.