The detectives who investigated the Act MP John Banks did not seek advice from the Government's key legal advisers over whether to charge him.
Banks, who was found guilty last week of knowingly filing a false electoral donation return in his failed 2010 Auckland mayoralty bid, will resign his seat in Parliament on Friday.
The case was first investigated by the police in 2012 but the decision was made not to prosecute.
The Crown Law Office, which provides legal guidance and is regularly consulted by the police, has confirmed that detectives did not seek advice over whether to prosecute.
It said it only became involved when a Crown lawyer took over the private prosecution that eventually brought Banks before the courts.
On Monday the Independent Police Conduct Authority said it was to consider whether it should investigate the police decision in 2012 not to prosecute John Banks.
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 Labour has called for an inquiry into the case and said there should be no question around whether the IPCA investigates.