Fresh allegations have been fired at ACT MP John Banks about a donation to his 2010 Auckland mayoral campaign which was listed as anonymous but made out for a specific amount.

Records show $15,690 was donated for radio advertising for his campaign, the Weekend Herald reports.

Labour MP Trevor Mallard is asking how an anonymous donor can donate the right amount of money to cover a specific bill while remaining unknown.

Although, he says, the funds could have been donated by the company that did the work.

Mr Banks' expenses sheet for the campaign show the work was carried out by The Radio Bureau, run by The Radio Network and RadioWorks.

The company did not return calls by the Weekend Herald.

Mr Banks is being investigated by police to determine whether he knew piracy accused Kim Dotcom was behind an anonymous donation to his campaign.

Dotcom put $50,000 into Mr Banks' campaign fund for the Auckland mayoralty in 2010, in two cheques made out for $25,000.

Candidates must declare the source of donations if they know where they came from.

Dotcom's former employee, who didn't want to be named, told the Dominion Post he personally wrote out the cheques on the account of Megastuff, the Megaupload founder's company.

Mr Banks has told Prime Minister John Key he didn't know who made the donations, and Mr Key has taken him at his word.

Mr Key is rejecting opposition demands for Mr Banks to be stood down from his ministerial portfolios while the police inquiry is under way.

Mr Banks has also rejected claims from Mallard that Dotcom organised a discount for him to stay at a hotel in Hong Kong last December.