UPDATED 3.52PM: Former MP and Auckland Mayor John Banks is calling the Crown's actions a serious breach of process and a gross misconduct.

The Court of Appeal has cancelled his retrial and directed he be acquitted on charges that he made a false declaration over his election campaign expenses during his failed 2010 mayoral bid.

Mr Banks told media that he's been to hell these past three years, and today he's back.

"Truth has prevailed at a huge cost to my family and with so much stress and so much oppression to myself."

A review of Crown Law processes looks likely following the case after Crown prosecutors were criticised for failing to disclose evidence to Banks' defence team ahead of an appeal hearing.

Attorney General Chris Finlayson read the judgement and indicated he was looking into it.

"In broad terms, the Crown carries the can as it were, but there are matters of concern Crown law will be wanting to look at.

"I suppose that's as far as I should go at this stage because I stayed right out of it.

Graham McCready brought the original case forward, before Crown Law took over the prosecution.

He believed the point was always to keep the legal system open and test the rights of New Zealand citizens to bring a case before the courts.

Despite the decision that saw Banks acquitted, Mr McCready was not disappointed with the outcome.

Mr McCready claims the case is a lesson for all politicians around the filing of electoral returns.

"I'm not emotionally attached to these cases. I'm not on any kind of crusade. I take them one at a time and take them as far as the law allows."

The acquittal ended an 11-month process after Mr Banks was found guilty of filing a false electoral return in June 2014.

Three weeks ago, Mr Banks sought urgent action from the court in Wellington.

He said the Crown knew but failed to declare that Kim Dotcom had changed his evidence over the donations when confronted with evidence collected by Amanda Banks.

She'd been so stung by the original guilty verdict that she hunted down fellow lunch guests at the meeting where the donations were allegedly discussed, getting them to sign affidavits saying that wasn't the case.

Mrs Banks was upset that the district court judge believed Kim Dotcom and his ex-wife Mona's evidence ahead of theirs.

Three Court of Appeal judges say the new evidence was reliable - and the Crown's decision to withhold it misled the court.