The Independent Police Conduct Authority (IPCA) says now John Banks' trial has concluded, it will consider whether it should investigate the police decision in 2012 not to prosecute the Epsom MP.
The ACT MP will formally resign from Parliament on Friday, after being found guilty at the High Court in Auckland last week of knowingly filing a false electoral donation return in his failed 2010 Auckland mayoralty bid.
After an initial investigation in 2012, police said there was insufficient evidence to proceed with some charges. Other complaints were laid too late after the return was filed for charges to be pursued. Private prosecutor Graham McCready then took a civil case against Banks.
The Labour Party said on Monday there should be no question around whether the IPCA investigates.
Justice spokesperson Andrew Little said others who looked at the decision after police thought there was a case to answer and there should be a high-level inquiry into the original decision.
Mr Little is critical of how police handled the Banks, case, and other alleged electoral breaches, and believed the IPCA should proceed without hesitation.
Warren Young, the IPCA's group operation manager, said the authority received two complaints earlier this year.
"We were awaiting the outcome of court proceedings. We indicated to the complainants that we would reconsider our position at that point. So we are now in the process of doing that, but we have not yet determined what further action, if any, we will take."
Decision to quit 'sensible'
Prime Minister John Key says he still believes that John Banks is an honest man, but concedes his resignation from Parliament is the right thing to do.
Banks could have stayed on as MP because no conviction was entered against his name last week, and his lawyer's request for a discharge without conviction was not due to be heard until after the end of the parliamentary term. MPs may then have had to be recalled to vote on whether a by-election should take place.
Mr Key told Radio New Zealand's Morning Report programme on Monday that Banks' decision to resign was sensible and gave him time to look at how he might potentially clear his name.
"This allows him to take a step away from essentially what would have been process that would have been certainly messy for the ACT party and potentially a little bit messy for the centre-right of politics," he said.
He said he believed Banks was an honest man, who he said had not exhausted the appeals process on his case.
Mr Key said his chief-of-staff had spoken to Banks over the weekend and he was "aware of our general thinking" but the decision to resign was one he led to a certain degree himself.
"He was aware obviously he had to resign. There's no way he was going to allow Parliament to be recalled in that intervening period when Parliament had essentially broken up for the year, but not formally dissolved, between the first of August and the fourteenth of August.
No enthusiasm for by-election
Opposition parties seem unlikely to push for a by-election in the Epsom electorate so close to the 20 September general election.
Mr Key said the Government intends to seek Parliament's support next week not to hold a by-election in the Auckland seat. The Government needs the support of 75 percent of MPs to avoid holding a by-election and looks likely to achieve this.
Labour Party leader David Cunliffe said his party was consulting internally and with other parties.
He said his personal, preliminary view is that the people of Epsom would not thank them for choosing to hold two elections in such quick succession, and it would be viewed as a waste of money.
However, Mr Cunliffe won't rule anything out and said Labour was likely be able to confirm its position on the by-election later on Monday.
The Green Party caucus will decide its position when it meets next Tuesday but, a spokesperson said, it was unlikely the party would seek a by-election.
Maori Party co-leader Te Ururoa Flavell said he still had to talk to his colleagues but did not think a by-election should be held.
Banks to quit Epsom seat on Friday
John Banks will write to Parliament's Speaker on Monday formally advising of his decision to resign. He has just days left as an MP after announcing he would vacate his seat at 5pm on Friday 13 June.
Banks revealed his intentions in a short statement on Sunday and did not outline why he had chosen to go. He said the timetable for his departure would allow constituency, administrative and staffing matters to be dealt with over the coming days.
"I have given my heart and soul over four decades to making a worthwhile contribution to this country. I have always endeavoured to do the right thing. Consequently I am deeply saddened at this turn of events."
ACT Party leader Jamie Whyte said he told Banks he thought he should resign but that it was his own decision to go. He said it would have been difficult for Banks to sustain his position if he had stayed, and he had done the right thing.
Friend and supporter Michelle Boag, a former National Party president, said she knew the MP was going to choose to leave his seat.
Ms Boag said he had made the right decision because there were many unresolved issues from the court case which he can now focus on. She said she didn't think Banks would ever seek public office again.