The police have referred Auckland Mayor Phil Goff to the Serious Fraud Office (SFO).
The SFO will assess the matter which relates to Goff's election expenses.
The SFO announced the news on Friday afternoon and said it will offer no further comment at this time.
In September 2019 a formal complaint was laid over Goff's 2016 election expenses declaration.
It related to the declaration of cash donations from fundraising auctions of $366,115.
Stuff reported the complaint was under section 112D of the Local Electoral Act 2001, 'Filing a false return of electoral donations and expenses'.
In January police announced they would take no further action against the complaint because the three-year-limit for prosecution had been reached.
A spokesperson for Goff said he had no knowledge of a complaint against him, and will make no further comment until he is contacted by the SFO.
Goff won a second term as Auckland's Mayor in October 2019. He defeated his opponent John Tamihere by a landslide, winning 155,957 votes to 70,000.
The investigation of Goff comes on the same day that Christchurch Mayor Lianne Dalziel was referred for her own election expenses.