Former National Party leader Don Brash is thanking police after their second look at the theft of his emails.

It comes nearly a year after he expressed disquiet over the way police handled their investigation into the emails, which were published in the book, The Hollow Men, by journalist Nicky Hager.

The review has confirmed the source who passed the emails to Mr Hager cannot be identified. It has also confirmed there were security flaws in National Party offices.

Dr Brash says the original police investigation lacked urgency and gave every appearance of being treated as a matter of no consequence.

After he formally complained to the Independent Police Conduct Authority in the middle of last year, Dr Brash says police launched a thorough investigation. He says all the obvious potential suspects were interviewed, some for the first time.

Dr Brash says even though police have failed to identify the source of the leak, he certainly appreciates the effort they have put into this second investigation.

Nicky Hager says he will never reveal how he obtained Dr Brash's correspondence. He says 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. Unfortunately many people fell for it and went running after the stolen email story."

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

"The fact is that the main way information comes out of organisations is not because the cleaner hasn't been cleared or something. It's because

people walk in and out of there. And they have different motives. And sometimes I think the public's got a right to know something which has been kept secret."

Mr Hager says that will never be stopped, no matter how many burglar alarms you install.