Bennett's response is here. In it she claims that the material was withheld because
At the time we were responding to your request, the independent review of New Zealand's National Action Plan was published, and public comment was being sought. Due to the active public consultation period, officials wished to avoid any potential for free and frank expressions of opinion to have an impact on the consultation, so that interested stakeholders could form their own view. Accordingly some sections of the documents that dealt with the independent review mechanism or aspects that were covered in the independent reviewer's report were withheld.
Which is interesting, given that the IRM report was released on 18 February, public comment closed on 3 March, and the material was not released until 17 March. I'd call Bennett a liar, except that she's just saying what her officials have told her to say. And its the officials who have something to hide here, because those "free and frank expressions of opinion" are pretty bloody embarrassing for them and demonstrate the government's continued bad faith around the OGP.
So what did they withhold? Firstly, in a February 2015 background briefing they state the obvious about our unambitious action plan commitments:
The risks are mostly reputational. Our OGP actions could be perceived as not reflecting our high reputation in this area. For example, action plans consisting of commitments that are existing programmes of work risk criticism for lacking ambition, and limited engagement with the public and stakeholders in their development, implementation and evaluation will lead to OGP and domestic criticism. As an existing high performing country, demonstrating stretch in action plans will always challenge New Zealand. Our view is that these risks will be managed by the proposed programme.
In September 2015, after they had received initial feedback on the mid-term self-assessment report, they realised they had a problem, complaining about a "vocal minority" (three quarters of their submissions) who took a "narrow view" that New Zealand should actually adhere to the spirit and rules of the Open Government Partnership and make ambitious commitments and consult properly over them, rather than have an action plan of existing programmes with no extra ambition. They noted that they "do not agree with this view", but that it could result in "negative comment" when the self-assessment was released.
By December, that had turned into full-throated denial. Their advice then predicted that IRM reviewer Steven Price would be critical of their action plan and accused him of "reflect[ing] particular civil society and community voices and their concerns". However, they thought there was "a very positive story of progress to be told about New Zealand's achievements", and they presented a communications plan which would attempt to discount the IRM report. In the end, they didn't do any of it - in fact, SSC did not issue any public comment on the IRM report at all, and refused to acknowledge it or its criticisms in any way.
Later in the submissions analysis in that document, they go into further detail on why their critics are wrong and how it is "legitimate" to use existing programmes as core OGP commitments. The IRM report should have disabused them of that notion. Unfortunately, their response has been to retreat further into denial, delaying our next action plan, while promising that public input will be ignored. But that isn't going to solve the core problem: the OGP demands real commitments, and real reform; trying to pretend it doesn't will eventually damage the very reputation SSC is predicating their inaction on.