Police are not charging Epsom MP John Banks over a complaint that he filed false donation returns in the 2010 Auckland mayoralty election.

In a letter to Labour MP Trevor Mallard, who laid the complaint, police said they could not establish Banks had the necessary knowledge that a $15,000 donation from SkyCity was recorded as anonymous before he signed and submitted his return.

They made the same finding about anonymous radio advertisements worth $15,690 and two $25,000 donations from internet million Kim Dotcom.

Investigating assistant commissioner Malcolm Burgess has confirmed this in a statement.

He said there is insufficient evidence to consider a prosecution for an offence under section 134(1) of the Local Electoral Act 2001.

"The inquiry has established the return was compiled by a campaign volunteer who assured Mr Banks it was accurate before Mr Banks signed and transmitted the return."

Burgess confirmed that in terms of Section 134 (2) of the Act, relating to transmitting a false return, there was a statutory limit of six months from the time of elections when complaints must be laid.

"Police first received the complaint for investigation on April 27, 2012, well after the expiry of the six month period. Police are therefore unable to consider charges pursuant to Section 134 (2)."

Police said the SkyCity donation, written out as a cheque to Team Banksie 2010, was handed to Banks in a sealed envelope.

It was subsequently recorded in the electoral return by the Banks' team treasurer as anonymous.

On the issue of the radio advertisements, police established that Banks had personally solicited a request for help from a donor.

That donor had asked to remain anonymous for the purposes of the police letter to Mallard.

As a result of the discussion, it was agreed the donor would help with a donation of radio advertising for the campaign.

It was recorded by the treasurer as an anonymous donation and also as an advertising expense of $15,690 in the expenses return.

In June 2010, Banks also personally solicited a donation from Dotcom. It resulted in him paying two cheques of $25,000 to the Team Banksie bank account.

Both were again recorded by the treasurer as anonymous in the electoral return.

Detective Superintendent Peter Read said the file was now complete.

Banks said today he wanted "to acknowledge the very professional and diligent way in which the police have approached this investigation, and the way they have interacted with me and members of my campaign team".

"We have always maintained that our 2010 electoral affairs were conducted within the law, and we are glad that these matters can now be put behind us," he said.

"I will be making no further comment about these matters."

A spokesman for Prime Minister John Key said: "The minister has always given an assurance that he's complied with the law, and I believe that's been confirmed by the police today."