When is a private dinner attended by a Cabinet minister no longer a private dinner? When the taxpayer is meeting some of the cost - either directly or indirectly.

Or so argued Labour MPs in Parliament yesterday as they pressed Judith Collins once more to reveal the name of the senior Chinese border control official with whom she dined in Beijing last October along with her friend Stone Shi, the founder of the milk exporting company Oravida, of which her husband is a director.

The Justice Minister's perceived conflict of interest may have been last month's story. But Labour is continuing to ask questions about her China trip because it sits well with the party's attempt to portray National as hostage to "crony capitalism''.

Citing advice from the Prime Minister's office, Collins responded to Opposition questions by continuing to insist the dinner was a private affair and it was not appropriate to disclose the name of the official who was a friend of Mr Shi.

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"He was attending a private dinner and is entitled to protection of his privacy.''

This did not wash with Opposition MPs who argued that she was obliged to name the official because taxpayers had funded the visit. With whom she had met or dined was therefore a matter of public interest.

Labour deputy leader David Parker suggested to the Speaker, David Carter, that question time in Parliament was an important element in ensuring ministers were held to account for such spending. Ministers should therefore take questions seriously and endeavour to give informative replies.

Carter, however, ruled that Collins had addressed the basic question Parker and his colleagues were asking. "The minister has answered by saying she cannot answer that question.''

Labour's Grant Robertson - who has been leading the Opposition offensive against Collins - said taxpayers were entitled to "have some honesty from the minister for once''.

But Collins retorted that it was a private dinner, "and as that member might one day find out, even ministers are allowed to have private dinners with friends''.

But that was not the issue yesterday. What was worrying Opposition parties was that Collins' refusal to name the official might end up setting a precedent.

Labour's Trevor Mallard asked whether Carter was making a new Speaker's ruling that a minister could decline to answer a question because it was in his or her private interest not to answer it.

But the Speaker assured Mallard - to Mallard's and other Opposition MPs' relief - that he was not saying that at all.