The police are to be investigated over their decision not to prosecute former ACT MP John Banks for knowingly filing a false electoral return.
The Independent Police Conduct Authority said on Tuesday it would hold an inquiry in the wake of Banks being found guilty of the charge which related to his failed bid for the Auckland mayoralty in 2010.
In 2011, police decided not to prosecute Banks over donations that had been listed as anonymous. A private prosecution was then brought by Graeme McCready from Wellington, and this was subsequently taken over by the Crown Law Office.
The case was heard before Justice Wylie alone at the High Court in Auckland. The judge found Banks guilty on 5 June this year, but has not formally convicted the former MP for Epsom as his legal team is seeking a discharge without conviction.
A week ago, the IPCA revealed that it had received several complaints from members of the public about the case and would determine whether a full investigation was justified. The authority has written to them, saying such investigation will now be carried out.
One complainant, Roger Brooking from Wellington, says he can't understand the police's actions.
"It just seemed blatantly obvious to me, based on simply what I was listening to on the radio and the television and reading in the newspaper, that it was an open and shut case. And then I just simply couldn't understand it that the police didn't prosecute."
Mr Brooking told Radio New Zealand's Checkpoint programme on Tuesday he thinks police may have thought it would have been embarassing to prosecute Banks as he was a member of the Government at the time.
Police said on Tuesday that they welcome the investigation. Banks' lawyer David Jones has declined to comment.
Graeme McCready has subsequently filed other complaints over the matter, including two against Prime Minister John Key at the Auckland District Court on Tuesday.
One alleges that Mr Key conspired to defeat the course of justice by ensuring Banks was not prosecuted by police; the other alleges Mr Key was an accessory to the offence by being wilfully blind to the contents of the police document.
John Banks resigned as an MP on 13 July at Parliament and is to be sentenced on 1 August.