Prime minister John Key has refused to sack minister John Banks over fresh revelations about his campaign donations.

Police files reveal Banks told lawyer Gregory Towers in February he couldn't help Kim Dotcom publicly because of his ''support'' for his election campaign.

In a sworn statement the internet mogul said he gave Banks' campaign two cheques for $25,000 - and in two phonecalls Banks confirmed he'd received the money.

JOHN BANKS: The centre of a police investigation into campaign donations.

His statement was backed by bodyguard Wayne Tempero.

Banks insists he doesn't know where the donations - which were declared anonymous - came from.

Police have said local electoral laws were broken but they didn't have enough evidence to press charges.

Opposition parties have called on Key to sack Act leader Banks, his minster for small business.

Labour's deputy leader Grant Robertson says the files prove Banks knew where the money came from.

But Key said today: ''Nothing's changed. He hasn't broken the law...there is no charge against him.''

He hasn't read the full report from police and accused Labour of a ''politically motivated attack.''

Key said in May that if Banks had lied to him he would be sacked. Robertson says Banks must now go because the statements from Dotcom, his bodyguard and his lawyer ''show that John Banks did know about the donation and could recall it as recently as February this year.

"John Banks lied to the Prime Minister's Chief of Staff and by the Prime Minister's own rules, he should be sacked.''

The police report proved he had broke the law over a donation of radio advertising, but could not be prosecuted because of a time bar.

"In other words, the law had been breached but John Banks got away with it because of the elapsed time. Whichever way the Prime Minister looks at it, John Banks has not met the standards to be a Minister, and he should be sacked," Robertson said.

In January, told media he hardly knew Dotcom after the millionaire was detained on US anti-piracy charges.

In fact he had met Dotcom a number of times, lunching at his mansion and riding in his helicopter. Dotcom says he gave Banks two cheques for $25,000 towards his 2010 campaign to be Auckland mayor.

Police yesterday opened files from their investigation into the donations from Dotcom and a $15,000 donation from SkyCity, also registered as anonymous.

A witness statement from Mr Towers reveals he contacted Mr Banks, by then MP for Epsom, on February 8, to ask for his help in getting an additional mattress for Dotcom's Mt Eden cell. Mr Banks called back the next day.

"John Banks said that as much as [he] wished to publicly support Kim that may backfire on Kim if it became known about the election support," Mr Powers said. "He said he was very supportive however of Kim's case."

A witness statement from Dotcom also reveals Mr Banks twice confirmed he received the donation by telephone. He has previously revealed Mr Banks called to thank him for the gift.