A senior police investigator is to review the Don Brash email case, after the former National Party leader suggested there may have been political bias or incompetence in the investigation.
Dr Brash on Thursday wrote to the Government requesting a Commission of Inquiry into the conduct of police over emails stolen from him.
The emails formed the basis of a book, The Hollow Men by Nicky Hager, about the National Party's 2005 election campaign.
Detective Inspector Harry Quinn, who led the first investigation, said police could not establish how the emails were stolen.
Dr Brash says a heavily-censored file on the inquiry he has recently received adds weight to his concern that there was a lack of diligence in the police investigation.
He says it took police months to talk to people who received the emails and a small amount of police resources was devoted to the investigation. He says that is inexcusable and creates a perception of incompetence or political bias.
Dr Brash wants a Commission of Inquiry to look at whether there was any police bias or political interference in the investigation, but does not want it to look at the theft of the emails.
Police Commissioner Howard Broad said on Thursday Dr Brash has continued to raise concerns about police handling of his case which Mr Broad says has the potential to undermine public trust and confidence in police.
The inquiry will be carried out by the Auckland Assistant Commissioner Steve Shortland. An independent adviser - likely to be a Queen's Counsel - will also be appointed after consultation with Dr Brash.
Dr Brash says he is pleased at the police review and will accept its outcome - as long as whoever conducts it has impeccable apolitical credentials.
Prime Minister John Key on Thursday ruled out a Commission of Inquiry, saying it would be too costly in terms of money and resources.
Mr Key says police have advised him that there were gaping holes in the security of Dr Brash's office when the emails were stolen, and acknowledges Dr Brash's frustration in not having the matter concluded.