Release the secret letter!

Stuff reports:

A National MP is calling on the government to release the contents of a letter about Let’s Get Wellington Moving he claims pushed the construction of a second Mt Victoria Tunnel back 10 years.
However, Associate Transport Minister Julie Anne Genter, who sent the letter to Minister of Transport Phil Twyford on March 26 this year, says the government has “nothing to hide” and the letter was being withheld in line with the Official Information Act and the advice of the Chief Ombudsman.
National MP for Hutt South Chris Bishop said Genter ran the risk of tarnishing her political legacy “as the Associate Transport Minister who stopped Wellington moving for a decade”.

This is a letter from the Associate Minister of Transport to the Minister of Transport on the Wellington transport programme. How can this possibly remain secret?

One can only conclude the releasing it would be explosively bad for one or both Ministers.