So why not release it then?

The Herald reports:

Prime Minister Jacinda Ardern says there is nothing explosive in the secret letter from Julie Anne Genter expressing her views on a multi-billion-dollar transport package for Wellington.

Then why not release it?

I agree it won’t be explosive. It will no doubt say that Genter is against any tunnels and any roads and wants them deleted from the programme or made lowest priority. This will not be a surprise.

Genter has refused to release the letter to protect free and frank discussions between ministers, and Ardern backed this up, 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.”

There is no precedent for refusing to release a formal letter from an Associate Minister to a Minister.