Wellington, May 22 NZPA - Author Nicky Hager believes a review of the investigation into how he got his hands on Don Brash's private emails will prove police did their best to find out.

Hager's book The Hollow Men, published in 2006, was based on hundreds of the former National Party leader's emails.

It revealed the secret inner workings of the party and caused an uproar.

Police launched an investigation and concluded it was unlikely anyone would be identified and charged unless there was an admission.

Dr Brash yesterday called for a commission of inquiry into the integrity of the investigation.

He believes the police did not make a serious attempt to find out who stole his emails.

Within hours Police Commissioner Howard Broad announced the investigation would be reviewed by Auckland Assistant Commissioner Steve Shortland with an independent adviser working alongisde him, appointed after consultation with Dr Brash.

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

Hager told NZPA the review was good news.

"There's been a campaign of casting doubt on the police investigation and it's worth everyone's while for someone independent to confirm that they did their best and there's no great conspiracy," he said.

"Dr Brash should have given up long ago and just understood that leaks happen, it's part of a democratic society. I wasn't going to reveal my sources and he's just going to have to live with that."

Dr Brash also welcomed the review.

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

"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.

Dr Brash said the independent observer needed to be politically neutral and independent.

`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."

NZPA