Labour is accusing Attorney-General Chris Finlayson of making false pecuniary interest declarations because he didn't say he was the director and shareholder of a company.
However, Mr Finlayson says he didn't declare it because he had no interest in the company, which was a corporate trustee incorporation for a friend's trust.
Every year since 2005, MPs have registered their pecuniary interests, such as the assets, debts and gifts they have accumulated or received, which are then published. The register provides a protection against arguments that a conflict might arise between a member's public duty and private interests.
Writing on Labour website Red Alert, MP Trevor Mallard said Mr Finlayson should have included the company on the register.
"(Mr) Finlayson appears to have signed a false statement every years since March 2006," Mr Mallard wrote.
Mr Finlayson said that since 2006 he had been a director and shareholder of Te Puhi Trust (2) Ltd with two other people. He said the incorporation owned no assets and only existed to be a trustee for a family trust which benefited a family and charitable causes.
"I have no pecuniary interest in any of these entities, as confirmed in a letter from the trust's lawyer today."
Mr Finlayson said he was checking with the registrar whether he should have included his role in the register and expected an answer tomorrow.
He released his lawyer Stuart Perry's letter. In it he wrote Mr Finlayson agreed to become a director and shareholder of the company at the request of Peter and Mary Biggs.
Mr Perry said the company existed "entirely for members of the Biggs family and for charity. You have no personal or pecuniary interest whatsoever in any of these entities."
Mr Perry is also a director and shareholder of the company, "the only difference being that I am the Biggs' solicitor and you are their friend".