Senior Cabinet Ministers were sounded out about an Auckland light rail bid from the NZ Super Fund in January 2018, months before a proposal was put to the Government.

But Transport Minister Phil Twyford says the subsequent proposal was unsolicited, and any suggestion that he has been lying about that is defamatory.

His position has been backed up by the fund and by Sir Michael Cullen, who was at the January 2018 meeting with Twyford, Finance Minister Grant Robertson and Sir Brian Roche, who is now chair of the NZ Transport Agency.

The Government is currently considering two bids for Auckland's light rail: one from NZ Infra, a joint venture between the NZ Super Fund and Canada's CDPQ Infra group, and one from the NZ Transport Agency.

The Labour Party campaigned on having light rail from the CBD to Mt Roskill built by 2021, but the project has been delayed until February next year when the Government announces its preferred option between the proposals.

Further questions have emerged about whether the Government wants a rapid transit system from the CBD to the airport, or a slower system with more stops that would help with urban renewal.

Documents released under the Official Information Act include emails from Will Goodwin, the head of the Super Fund's NZ direct investments team, to Twyford's office about the January 2018 meeting.

"It was really useful to gain a better understanding of your current thinking and also talk through the Public Partnership infrastructure model NZ Super has been developing," Goodwin writes in his email, sent in March 2018.

Twyford said there was nothing unusual about the meeting, and it was wrong to suggest that it showed the Government has asked for the NZ Infra bid.

"It is true there were conversations about the bid before it was made public, that is normal in Government, but that is quite different to the Government soliciting it.

"NZ Infra's proposal was not solicited by the Government. It would be wholly inaccurate to suggest that."

In a statement, the NZ Infra backed up Twyford.

"Prior to submitting our proposal, we discussed it with Sir Brian Roche and Sir Michael Cullen and, together with them, corresponded with and sought meetings with Ministers to explain the concept and our investment.

"There was no invitation from the Government to submit such a proposal nor any pressure to do so."

Cullen said that the Super Fund had initiated the approach.

"The purpose of the January meeting was to try to ensure that the Government was aware that the Super Fund was interested in putting forward a proposal and that space was made to listen to that proposal."

Twyford has subsequently appointed Roche to chair NZTA, but Twyford said there was no conflict of interest "because Sir Brian is not a decision-maker on the light rail project".

National Party transport spokesman Chris Bishop has criticised the process.

"Two years after the Prime Minister made this flagship promise, we have no business case, we have no costings, we have no route map, we don't even have an identification of the objectives of why they're doing it in the first place.

"If the Prime Minister had the courage of her convictions, she would remove Phil Twyford from the portfolio."