Former National Party leader Don Brash says he appreciates police efforts to find who was behind his emails being given to a journalist, but accepts the culprit will probably never be found.

Police today released findings of a review they did into how they handled a investigation into the emails which were published in the book, The Hollow Men, by journalist Nicky Hager.

The review found the original investigation was competently carried out and its findings were correct. However, the review was critical of the lack of management over the timeliness of progress reports provided to Dr Brash and that he had to wait too long for his copy of the final report.

The review team spoke to nearly 200 people; parliamentary employees - including IT, security, messengers, cleaners and contractors, and others to confirm information obtained in the first investigation.

The review, overseen by Auckland Region Assistant Commissioner Steve Shortland, investigated complaints and allegations made to Police Commissioner Howard Broad and the Independent Police Conduct Authority by Dr Brash that the original investigation was conducted in a "cavalier fashion" and was the subject of political bias.

Dr Brash welcomed today's review findings that the police could have acted faster.

"The original police investigation of the breach of security around my parliamentary office lacked urgency, and gave every appearance of being treated as a matter of no consequence," Dr Brash said.

"Following my formal complaint to the Independent Police Conduct Authority in the middle of last year, the police launched a thorough investigation, interviewing all the obvious potential suspects, some for the first time. Even though their inquiries have failed to identify the source of the leak, I certainly appreciate the effort they have put into this second investigation.

"At this stage, it appears unlikely that the person who stole emails from my office will ever be identified and I greatly regret that. However, I have moved on to new challenges and, though the report from the IPCA is still to come, I am willing to accept that there is nothing more that the police can do," Dr Brash said.

Nicky Hager said he will never reveal how he obtained Dr Brash's correspondence.

He said it has been his position all along that claims of theft were a diversion by those who were embarrassed at the information he received.

"The reason they did that is it took attention off all the other MPs who are now running New Zealand - they are now the Government," he said.

"Unfortunately many people fell for it and went running after the stolen email story."

Mr Hager said any organisation can have security holes, but that is not the main way information gets out.

Mr Shortland said the findings of the review did not support the complaint that the investigation was conducted in a "cavalier fashion".

However, it was of the view that there had been a lack of urgency to complete the investigation once it was established that there were no threats to national and parliamentary security.

"The original investigation was conducted in accordance with police practice," he said.

"However, I acknowledge we could have been more timely with the closing stages of the investigation and final reports. But this in no way detracted from the professional conduct of the staff involved or the final outcome of the investigation."

Mr Shortland said issues involving police management and administration of the case file had been addressed and guidelines for the receipt, evaluation and assessment, investigation and monitoring of complaints of a political nature were being developed."
There was no evidence of political bias.

Although no suspect leads were identified, police interviews showed unsatisfactory security on the third floor of Parliament in terms of access to the floor and offices and to individual computers.

"Despite the number of staff and hours invested in the investigation and the review and examination of computers, it has not been possible to establish the source of the emails and other documentation that found there way into the public arena," said Mr Shortland.

"I am fully satisfied that everything possible has been done to identify the source of the emails and this is now the end of the matter."