Labour is urging Prime Minister John Key to ask police to release the entire file on the investigation into the theft of former National Party leader Don Brash's emails.

Dr Brash this morning called for a Commission of Inquiry into the investigation of the theft of his emails.

He has written to Mr Key and Police Minister Judith Collins formally requesting the commission look into the integrity of the police investigation and police behaviour since publicly announcing the investigation was closed.

Hundreds of his private emails found their way into the book The Hollow Men: A Study in the Politics of Deception, written by Nicky Hager, which contained information from emails supposedly leaked by National party members.

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

"There has been much rumour and innuendo, mostly from National Party sources, about who leaked the emails. John Key now has the opportunity to end this speculation," said Labour leader Phil Goff.

"One of the quickest ways John Key can do this is to ask the police to release the unedited file of its investigation."

A spokesman for Ms Collins this afternoon referred calls to Mr Key, and a spokesman for Mr Key said he had no immediate comment.

Dr Brash said there were important issues relating to every New Zealander's privacy and the integrity of our political system that deserved resolution.

"Everybody has a right to expect their correspondence will not be illegally intercepted or read by people it is not intended for."

In a democracy everyone had an interest in being assured that the police take such issues seriously, he said.

"The heavily-censored information that was released to me and some media last week has added significant weight to my concern, held since I was briefed by police in mid 2007, that the investigation into this matter lacked any sense of urgency or diligence."

He said police conduct since closing the investigation - or declaring it "inactive" - was of concern and their behaviour was highly unusual.

Despite promises in April 2008 that he would receive the final report on the investigation, he received it only a year later.

Any suggestion of political bias had to be addressed seriously, Dr Brash said.