The NZ Transport Agency has ordered an independent investigation to uncover the leaker of confidential information about Auckland's light rail project.

Photo: RNZ / Alexander Robertson

Multiple documents and letters relating to Labour's transit proposal have been leaked to media in recent weeks, raising questions about the much-delayed project.

The agency's board has appointed Michael Heron QC to investigate how the papers got out and to report back before the end of the year.

He'll also review the agency's policies and practices relating to sensitive material and recommend improvements.

"It is disappointing and concerning that this information has been improperly disclosed, and it is appropriate that an independent investigation be undertaken in order to establish the facts," board chairperson Sir Brian Roche said in a statement.

Sir Brian declined to make any further comments while the inquiry is underway.

Earlier this month, Sir Brian acknowledged the agency had dropped the ball in its early handling of the project.

NZTA had been tasked with sounding out potential options for funding and building the network, but the process was delayed by an unsolicited bid from the NZ Super Fund.

Then-interim NZTA chair Nick Rogers reportedly dismissed the Super Fund bid, describing it as little more than a six-page powerpoint presentation.

But Transport Minister Phil Twyford insisted that the alternative proposal be properly considered and transferred the responsibility to his Transport Ministry.

The Ministry is now considering two separate proposals - one from NZTA and a joint venture with the Super Fund and CDPQ Infra, NZ Infra.

The promise to build light rail from downtown Auckland to the airport within a decade was made by Jacinda Ardern in her first policy announcement and campaign rally as party leader, ahead of the 2017 general election.