Former National Party leader Don Brash has welcomed a high-level review of the police investigation into how his private emails ended up in a book.

Police Commissioner Howard Broad announced the review yesterday, after a call by Dr Brash for a commission of inquiry.

Dr Brash wrote to Prime Minister John Key and Police Minister Judith Collins asking for a commission to look into the integrity of the police investigation and police behaviour since they publicly announced the investigation was closed.

Hundreds of Dr Brash's private emails were used by Nicky Hager in his book The Hollow Men: A Study in the Politics of Deception, published after the 2005 election.

A long police investigation concluded it was unlikely that anyone would be identified and charged "unless someone makes an admission during the interview process".

Commissioner Broad said continued questioning of the police role could undermine public trust and confidence in the force.

He ordered a full review of the case, including the recent release of the highly edited file, to be conducted by Assistant Commissioner Steve Shortland of Auckland, with an independent adviser working alongside him.

Mr Broad said the review would ensure that all practical avenues of investigation concerning the case were exhausted.

"If a contrary view is arrived at, I would expect the assistant commissioner to advise what further investigative work might be undertaken to bring the matter to resolution."

Dr Brash said he was happy with the steps being taken.

"I am very pleased at the police commissioner's reaction," he said.

"One of my major concerns was that the way the investigation was initially conducted suggested either incompetence or political bias and I think that's dangerous for the police and not good for the country."

Dr Brash said the senior appointment backed by the independent adviser to review the case went a "very long way towards satisfying my concerns".

He was prepared to stop pressing for a commission - for now. He said the independent observer needed to be politically neutral and independent.

Hager said Dr Brash's allegations against police were "incredible".

"I think it is extremely insulting of Don Brash to even suggest the police would cover up crimes.

"What he is suggesting is there are professional high-ranking police officers who might know about a crime and be prepared to lie about it and cover it up, a crime against the current Government."

Hager said he was confident his sources were safe but thought a commission would be a waste of time and money.

Police had interviewed him for their inquiry. "My impression was that they did a very thorough job."

National had an internal investigator and hired a private investigator and got nowhere, Hager said.

Dr Brash, when asked if he was concerned about bringing the integrity of police into doubt, said it was not him doing that.

"I think the way in which the investigation was conducted raised very serious concerns."

Dr Brash said the heavily censored police file released last week had added to his concerns and police conduct since they declared the investigation "inactive" was also concerning.

Despite promises in April last year that he would receive the final report on the investigation, he received it only a year later.

"I think the feeling was quite widespread that the initial investigation was either incompetent or biased or both," he said.

"I think the police commissioner is moving to put the country's concerns to rest in this way."

Mr Key yesterday said he understood Dr Brash's concerns but a commission of inquiry was a very expensive option that would divert police resources from fighting crime.

Police had found "gaping holes" in the security of the former Leader of the Opposition.

"The issue is not that the police want to reach a successful conclusion, it's that they've been unable to provide a successful conclusion," the PM said.

Speaking before being made aware of Mr Broad's announcement, Mr Key suggested Dr Brash take his "serious allegations" against police to the Independent Police Conduct Authority.