The Cabinet Office refuses to say who participates in them. According to them, we have no right to know who is making decisions at the highest level of our government.
This is absolutely absurd. The membership of some of these committees is already known. For example, the Officials Committee for Domestic and External Security Co-ordination has its own Wikipedia page, which lists its membership as
the chief executives of the Ministry of Foreign Affairs and Trade, the Defence Force, the Ministry of Defence, the Security Intelligence Service, the Government Communications Security Bureau, Police, the Ministry of Civil Defence and Emergency Management, the Treasury and others.
A publicly available document on New Zealand's National Security System [PDF, p. 23] makes it clear that those "others" - actually core members of the committee - are the Chief Executive of DPMC and the State Services Commissioner, and that the intelligence agency chiefs are not core members but attend
when matters are being considered that impact upon their priorities, plans, outputs and resources, except where a conflict of interest arises, or the Chair elects to hold a ‘members only’ session.
In other cases, we can make a pretty good guess. For example, it is highly likely that the Chief Executive of the Ministry of Business, Innovation and Employment sits on the Officials Committee for Economic Growth and Infrastructure, and that the Secretary of the Treasury sits on the Officials Committee on State Sector Reform and Expenditure Control. But we shouldn't have to guess. These people are not junior public servants whose identities must be protected in the name of a neutral public service. They are senior officials whose identities are publicly known and who can be expected to be both accountable and responsible. They are doing public business, and that fact should be publicly available.