Prime Minister John Key is refusing to release advice from the Cabinet Office that he says clears the Justice Minister of breaching Cabinet rules.
Judith Collins is under pressure from the Labour Party about a visit to her husband's milk export company Oravida when she was in China last year.
Labour says the visit was a clear conflict of interest because photos of it carried a message saying Ms Collins was full of praise for the milk, and that kind of endorsement is against Cabinet rules.
Mr Key told reporters on Thursday he won't release the advice he has received on the matter.
"We have a set of rules which are always just a guideline anyway. We look to follow those guidelines as best we possibly can and because questions were asked I did what you'd expect me to do which was seek advice from the Cabinet office and the advice was absolutely unequivocal - there was no breach of the Cabinet manual."
Mr Key said parties in Opposition claim the rules are really strict, but when in Government they claim something different.
Meanwhile, Labour failed to get more answers from Judith Collins about her trip because she wasn't at Parliament on Thursday.
MP Grant Robertson was denied leave by National members to postpone his question to Ms Collins until she was back in the House, so Attorney-General Chris Finlayson answered on the minister's behalf.
Mr Robertson asked who invited Ms Collins to visit the company; whether it came from her husband who is a director of Oravida, a golfing companion of Mr Key's who donated $55,000 to the National Party, or party president Peter Goodfellow, whose company is a supplier to Oravida.
Mr Finlayson replied he doubted it would be any of them, but he would find out.