Former National Party leader Don Brash accepts there is nothing more police can do about finding those responsible for the theft of hundreds of his emails.
Police launched a review of the original investigation after Dr Brash complained it had been conducted in a cavalier manner.
The review has found that it was carried out competently.
Assistant Commissioner Steve Shortland reviewed the police's handling of the case, following Dr Brash's suggestion there might have been political bias or incompetence.
The original inquiry found problems with the security of Dr Brash's office at the time, but was unable to identify the person who took the emails.
Mr Shortland says his review endorses those findings.
However, he concluded that once police established there was no threat to national or parliamentary security, their investigation displayed a lack of urgency.
Mr Shortland says the final stages of the investigation, including keeping Dr Brash informed of progress, could have been handled better.
Prime Minister John Key says the identity of the person who stole Mr Brash's emails looks set to remain a political mystery.
Mr Key says he still believes the parliamentary services computer system was hacked into, rather than the theft being an inside job.
