Prime Minister John Key says government minister Pansy Wong was following the law in the way she witnessed one of her husband's business deals in China.
The Labour Party says because Mrs Wong signed the document while on a private trip to China she should not have used her ministerial title, as she could be seen to be furthering her husband's business interests.
Documents released by Labour show that in January last year the National MP and Minister for Minister for Women and Ethnic Affairs acted as a witness to an agreement involving two hovercraft companies.
Her husband is a director of one of the companies.
Mr Key told Morning Report on Wednesday the advice from the Cabinet Office is that there is nothing in the rules against a minister witnessing a document.
Mr Key says in doing so, it is a legal responsibility that Mrs Wong should accurately record her occupation.
But Labour leader Phil Goff says it is clear Mrs Wong should not have signed the document as a minister and Mr Key should tell her she was wrong to do so.
Mr Goff says the Prime Minister is treating Mrs Wong differently than former National minister Richard Worth, who was rebuked for travelling to India on private business when a minister.