The Serious Fraud Office (SFO) has closed its investigation into Auckland mayoral election donations.

The investigation focused on fundraising donations made in 2016 and 2019 after the SFO said it received a referral from Police over mayor Phil Goff's election expenses in February 2020.

"The SFO takes allegations of electoral funding fraud very seriously," SFO acting director Paul O'Neil said in a statement today. "We have completed our investigation and we are satisfied that this matter can now be closed."

In a statement, the Phil Goff for Mayor campaign said it welcomed the SFO's decision.

"We are pleased that having completed the investigation, the SFO is satisfied that this matter can now be closed," a spokesperson said.

"The campaign has always been confident about the integrity of our procedures and that the rules governing donations were always followed."

Goff will not be seeking re-election this year.

The SFO said it had no further comment to make about the investigation.

While the SFO did not disclose the nature of its inquiry, Goff's 2019 campaign director Shale Chambers said the issue appeared to be the use of auctions in the 2016 and 2019 campaigns.

In September 2019, electoral officer Dale Ofsoske passed a complaint about Goff's 2016 election expenses to police over a $366,000 auction declaration, which included a book being sold for $150,000.

The book had belonged Goff - a former Minister of Foreign Affairs - and been signed by Chinese President Xi Jinping.

The SFO has also investigated Christchurch City Council election donations but in December 2020 closed its inquiry.

It focused on campaign donations made to mayor Lianne Dalziel in 2013, 2016 and 2019.

The SFO said it found no evidence of any criminal conduct during its investigation.

The agency is also currently prosecuting cases over donations made to the Labour and National parties and the New Zealand First Foundation.