Chief Ombudsman Peter Boshier has ruled Transport Minister Phil Twyford and Associate Minister Julie Anne Genter were correct to withhold a controversial letter.
Boshier said he found that allowing the ministers to withhold the letter was within their rights to have "free and frank expression of opinions during the policy process".
He said the ministers agreed with his suggestion that they issue a clarifying statement about the context and the content of the letter, which they have done.
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"We supported releasing a statement to put to rest the false speculation whipped up by opposition MPs about the letter's contents," Genter said in a statement on Wednesday.
Genter, a Green MP, came under fire in August after refusing to release a letter to Twyford on 26 March about the $6.4 billion Let's Get Wellington Moving (LGWM) transport package announced in May.
The package proposed integrating modern rapid transit, walking and cycling upgrades, and an extra Mt Victoria tunnel.
National's transport spokesperson Chris Bishop raised concerns at the time about the proposed completion date of the tunnel, alleging Genter convinced Twyford to push back the date.
The original proposal planned for a second Mt Victoria tunnel to be built between 2024 and 2029.
But the LGWM proposal announced by the Government after it agreed to adopt the plan had the completion date pushed back to between 2030 and 2034.
Genter has now confirmed she did raise concerns about the second Mt Victoria tunnel in the letter to Twyford, pointing to the proposed sequence of projects in the package.
Genter agreed to support the package if "a number of matters were clarified including that the public transport, and walking and cycling components would be completed as soon as practicably possible".
Genter also wanted to ensure "work on rapid transit be prioritised ahead of the second Mount Victoria tunnel".
The indicative package, ultimately approved by Cabinet, included this sequencing of the projects.
However, Genter denies allegations by some Wellington city councillors who reportedly said she threatened to resign if her demands were not met.
"I have not threatened to resign... I don't know why you would take the word of councillors over myself," Genter said in August.
The statement issued on Wednesday reiterates this: "The letter did not issue any ultimatums nor threaten a resignation."
The Ombudsman's ruling
Boshier's investigation followed a request from Wellington-based National MP Nicola Willis who asked him to look into whether the letter could continue to be withheld.
The letter to Twyford had a ministerial letterhead and Genter signed it in her capacity as Associate Minister of Transport.
The Chief Ombudsman said since both the Associate Minister and Minister of Transport hold the letter in their official capacities, it meets the definition of 'official information' under the Official Information Act.
But he said withholding the full letter is necessary to "maintain the effective conduct of public affairs through the free and frank expression of opinions between participants in the policymaking process".
He said public interest does not outweigh the need to withhold the full letter, but thought it best to require the ministers to issue a clarifying statement about the letter to meet the public interest in this case.
"I have given very close consideration to this case because of the fundamental issues involved here," Boshier said.
"They are of great importance to New Zealanders, involving a balance between the principle of availability of information and the effective conduct of public affairs."
Willis tweeted about the Chief Ombudsman's decision on Wednesday, saying it "reveals Genter demanded delay of the Mt Vic tunnel. And got it".
Bishop had a similar view, saying Wellingtonians "now know it was Julie Anne Genter and the Greens that delayed the second Mt Vic tunnel in a dodgy deal".