Public interest immunity to protect collective cabinet responsibility

In recent Senate Estimates hearings for the Department of Prime Minister and Cabinet, amid questioning of Minister Ludwig and officials about the Government's claim that no special deal had been offered those on board the Oceanic Viking, Liberal Senator Ronaldson made a concerted effort to get the names of those who attended a meeting of the Cabinet border protection committee where decisions on the matter were taken, in particular those from the Prime Minister's office.  The Minister refused, claiming public interest immunity and providing this statement of reasons (Finance and Public Administration 08/02/2010 at F&PA74).

"Disclosing the identity of ministers who attend cabinet meetings or cabinet committee meetings—I will deal with this issue first—would be contrary to the public interest because it would tend to undermine, as I indicated earlier, the collective responsibility of cabinet. It does that by inviting speculation about the collective basis of agreed outcomes, and this is consistent with the practice of successive governments. The relevant staff may attend meetings of the cabinet and its committees and working groups to provide advice when required depending on the subject matter under consideration, but they are not decision makers. Disclosing the name of ministerial staff who were present at particular cabinet meetings or cabinet committee meetings would also tend to undermine the collective responsibility of cabinet."

Senator Ronaldson responded that the names of members of  cabinet committees were public knowledge ( they are in the Cabinet Handbook, but remember this from earlier last year ?), called the Committee into a couple of closed meetings where it decided not to press the matter further, and finally said he may ask the Clerk to investigate. Given the fact that Labor and Liberal senators who sat on the committee have recently recommended against an independent arbiter proposal to deal with these sort of stand-offs, the issue won't run too far.

Revealing names of ministers in the room at a particular meeting would reveal also who was not there, so this must be what the Minister meant by the line invite "speculation about the collective basis of agreed outcomes." And the name of ministerial staff present could maybe suggest the subjects being discussed or angles being considered, or indicate whether that minister was or wasn't present. Still it seems something of a stretch to suggest disclosure "would..tend to undermine the collective responsibility of cabinet."