Cabinet minister Pansy Wong is being accused of improperly using her ministerial title to support her husband's private business contract, which has gone sour and is the subject of legal action.
Mrs Wong witnessed a contract between Pacific Hovercraft NZ and China-based Lianyungang Supreme Hovercraft, signed in April 2008.
Her signature is on a deed of variation, signed during a private trip in January last year, where she wrote her occupation as "Minister of NZ Govt" and her address as "Parliament Buildings, Wellington, NZ".
The agreement is signed by Sammy Wong, Mrs Wong's husband, who is stated as a co-director of Lianyungang Supreme Hovercraft.
A spokesman for Prime Minister John Key said Cabinet Office advice had been sought and Mrs Wong had done nothing wrong.
"She is a minister of the Crown. That is her occupation."
But Labour MP Pete Hodgson said Mrs Wong should have signed the document as an MP, not as a minister.
"If I'm witnessing that I've seen your photograph and there's a likeness to the person in front of me, I would write 'MP, Dunedin North,' because it's not a ministerial function.
"That is the mistake she's made."
He said the document created the impression the Government supported the contract, which raised conflict of interest issues.
Mr Hodgson said he would not have any problem with the document if Mrs Wong had written "MP for Botany" and used her residential address.
The Cabinet Manual, which outlines how ministers and MPs should behave, does not have any specific rules around signing documents as a witness.
A spokesperson for Mrs Wong said the minister had done nothing wrong.
"The [Cabinet Office] advice is that there's nothing wrong with witnessing a document like she did, and the Cabinet Manual wasn't breached.
"This is just Pete Hodgson trying to muck-rake and smear."
Mr Hodgson raised the issue in Parliament yesterday when he asked if Mrs Wong was acting in her ministerial capacity when she signed the deed.
Mrs Wong replied: "I was not aware that I signed a document overseas in my capacity as Minister for Women's Affairs, when it had nothing to do with the ministry."
He later released a Duncan Cotterill Lawyers letter, dated December 2009, claiming a breach of the contract and asking Lianyungang Supreme Hovercraft for payments over $800,000.