Under pressure in an election debate, Wellington Mayor Justin Lester says he supports the release of Julie Anne Genter's secret letter.
The move by the incumbent, who's running on the Labour ticket, is offside with Transport Minister Phil Twyford.
Genter, The Associate Transport Minister and Green MP, sent the letter in March about the Let's Get Wellington Moving project.
The refusal to release it has heightened speculation over how much influence the Greens had over the $6.4 billion project, in particular the decision to push back construction of Wellington's second Mt Victoria Tunnel.
Lester previously would not be pressed on whether he supported the letter's release, saying he wasn't interested in a "political spat" between the Opposition and the Government.
But the issue was raised again at a Wellington Chamber of Commerce mayoral debate this evening.
Candidates were asked whether they supported the letter being released.
Lester joined those who raised their hands.
After the debate, Lester said he supported the letter's release because he suspected there was nothing in it.
"I would be happy for it to be in public, but it's not my call."
Lester said its release could possibly help put rumours to bed, but understood it could set a dangerous precedent.
Twyford has previously said the letter should not be released because governments did not generally release correspondence between ministers, a convention they didn't intend to change.
Lester joins Greater Wellington Regional Council Sustainable Transport Committee deputy chair Daran Ponter in breaking ranks with Twyford.
"Let's see the evidence of that. Let's see what it is that she had pushed for and the direction that she has changed, because the things that have arrived on Wellingtonians' plate in relation to Let's Get Welly Moving are certainly not the things that they identified as projects they wanted when it went into the parliamentary process", Ponter said last month.
Genter has refused to release the letter to protect free and frank discussions between ministers, and Prime Minister Jacinda Ardern has backed this, saying its release would set a dangerous precedent.
"We have to be able to, as ministers, have discussions and policy discussions amongst ourselves before decisions are made, and if all of that's exposed all of the time, it makes it harder for us to do our jobs."