The Serious Fraud Office is looking into Auckland Mayor Phil Goff's election expenses.
In a statement released this afternoon, the SFO said it had received a referral from police in relation to the expenses.
The SFO said it would be assessing the matter and has no further comment at this time.
The news appears to have come as a shock to Goff.
His office says he has no knowledge of a complaint being referred to the SFO - nor of any irregularities.
He won't be making any further comment until his office has been contacted by the SFO.
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During last year's local body elections a formal complaint was laid over the 2016 election expense declaration filed by Goff.
The complaint related to the 2016 declaration of cash donations from fundraising auctions of $366,115. It is not known if this complaint is related to today's announcement by the SFO.
The complaint was handed to Detective Inspector Scott Beard.
The SFO also received a referral from police over Christchurch Mayor Lianne Dalziel's election expenses.
In December, the Christchurch City Council's electoral officer was asked to investigate why Dalziel had only revealed the identity of six people, who each donated more than $1500 to her campaign.
They donated through an auction her husband ran. She only declared the total amount that came from him.
Dalziel blamed her husband for the lack of transparency, saying he had believed the donors who gave more than $1500 did not need to be declared, and she had acted on his advice.
Former Christchurch mayoral candidate John Minto was one of two people to make a complaint about Dalziel's reporting of her expenses and said he was very pleased the matter was being taken seriously.
"I think in principle it's not much different to what NZ First are alleged to be doing, and the National party.
"So I think the police have decided to put them all together."
Dalziel is yet to comment on the police's decision.
Electoral law expert Professor Andrew Geddis of Otago University said a referral to the SFO is not the same as an investigation by the SFO. It will now assess the police file and see if there is enough in it to justify it opening an investigation, he said.
Geddis said even if the SFO does open an investigation and eventually lays charges, which is a fair way down the road, there is nothing in law to require a Mayor to step down.
That was the case with MP Jami-Lee Ross and recent cases of local body members remaining in office when charged with criminal offences, he said.
"Whether a mayor would lose office if convicted depends upon what they are convicted of.
"If the offence carries a potential jail term of two or more years, then their office is automatically 'vacated'," he said.
- additional reporting RNZ