An emotional John Banks has paid tribute to his wife Amanda whose "outstanding detective work" led him to being acquitted of filing a false electoral return.

"She is the hero in this piece of drama," said Mr Banks. "She was described by the judge as an unreliable witness. But she found the witnesses who would deliver the truth.

"The moral of the story is never underestimate the tenacity of a woman whose honesty and integrity is insulted."

His comments came at a press conference in the Stamford Hotel in Auckland, which was emotional at times, just a few hours after the Court of Appeal ruled he should not stand trial for a second time. The decision means he was acquitted of filing a false electoral return.

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The judgment was a sensational reversal of its previous decision to order a retrial, following the late disclosure of a document which Mr Banks' lawyer David Jones QC said "contradicts all evidence given at trial" by the Dotcom witnesses and made the prosecution "untenable".

Mr Banks said he had been to "hell and back" over the past three years, since Graeme McCready took a private prosecution against him.

"You've got no idea how punishing this has been... you've got no idea how lonely this has been," he told waiting media with a cracked voice.

"But if you believe in yourself and the truth, then honesty will prevail. And it has."

The decision today came after a urgent hearing in Wellington three weeks ago, where Mr Jones told the three appellate judges that Kim Dotcom's new statement was a "fantasy and a nonsense" and the Crown made a "very serious error" by not disclosing the new material, known as the Butler memorandum, ahead of a previous Court of Appeal hearing last October.

John Banks on his way to a stand up media conference at the Stamford Plaza, Auckland. The Court of Appeal has sensationally reversed its previous decision to order a retrial. Photo / Brett Phibbs
John Banks on his way to a stand up media conference at the Stamford Plaza, Auckland. The Court of Appeal has sensationally reversed its previous decision to order a retrial. Photo / Brett Phibbs

The case largely hinged on the credibility of who was right about a contentious lunch at the Dotcom mansion. After Mr Banks' wife Amanda unearthed new witnesses who corroborated her version of events, Dotcom said there were now two lunches.

In response for the Crown, Solicitor-General Mike Heron, QC, said the retrial should stand and the new evidence was a matter for the trial judge to decide.

But the Court of Appeal - Justices Ellen France, Forrest Miller and John Wild - disagreed with the Crown's argument that the "Butler memorandum" did not need to be disclosed.

"We hold rather that the Crown could not both withhold the memorandum and resist the appeal in the manner that it did. The effect was to mislead the Court," wrote Justice Miller.

"We are satisfied that there has been a serious error of process. It is, we accept,
attributable to an error of judgment rather than misconduct."

Mr Banks said he would take further advice about his legal options, but was scathing about the conduct of Crown prosecutor Paul Dacre, QC, and Solicitor-General Mike Heron, QC.

"I was held to account when I was told by the judge, 'the prisoner can take his place in the dock'. I was held to account...I'm saying the Solicitor-General has a lot to answer for," said Mr Banks.

"Withholding evidence from the Court of Appeal is a serious breach of process. Having the evidence in the briefcase but, for whatever reason, not giving it to the court is a very serious error of judgment at the very least.

"We wouldn't be here today if the Court of Appeal had that new evidence in front of them last year."

Mr Banks said the prosecution had cost him his job as the MP for Epsom and a Minister for the Crown, hurt him financially and caused stress to his marriage.

"I have no regrets. I hold no malice. Of course I'm angry, but these things happen. If you're going to have a full life, you're going to have trouble. and I reckon I've had my share of trouble."

Asked whether he would stand for election as Mayor of Auckland, Mr Banks did not rule out putting his hat in the ring.

"Isn't that where it all started? That's for another day...I've got to catch my breath."

But he did rule out attempting to resurrect his career in Parliament.

"I'm not sure if there's any political party that would want me."

The parliamentary career of the former Act leader ended after he was convicted in the High Court last year for not disclosing two $25,000 donations from internet mogul Kim Dotcom to his Auckland mayoralty campaign in 2010.

The Court of Appeal then quashed the conviction after the detective work of Mrs Banks, who was stung by the trial judge's opinion of her credibility when he preferred Mona Dotcom's evidence about a lunch, which was crucial to the case.

Mrs Banks tracked down two witnesses from America who corroborated her version of events and whose evidence the three appellate justices said was "likely to have changed the outcome of the trial", if accepted.

Read the full judgment here:

A retrial was set for July but Mr Jones applied to have the charge dismissed and also asked the Court of Appeal to reverse its decision to order a retrial following the discovery of the "Butler memorandum" which the Crown failed to disclose before the appeal hearing.

The undisclosed material was a memorandum written by lawyer Rowan Butler who was instructed by Mr Dacre to interview Kim Dotcom about the affidavits filed by the two American businessmen before the appeal hearing.

The pair said they arrived in New Zealand on June 5, 2010 and were taken to Dotcom's Coatesville mansion, where they had lunch with Mr and Mrs Banks and Dotcom and his wife Mona. Nothing about electoral donations was discussed, according to their affidavits.

This is at odds with evidence given at the trial, where the Crown contended the lunch was held on June 9, 2010 and the presence of the Americans was denied by the Dotcoms, as well as their bodyguard Wayne Tempero.

The defence was able to prove at the trial there was no lunch on June 9, because Mr Banks was campaigning and Mrs Banks was at work.

In finding Mr Banks guilty, Justice Edwin Wylie said Dotcom was a good witness but he was wrong about the date of the lunch and ruled it must have happened on June 5.

But when interviewed by Mr Butler about the new affidavits before the Court of Appeal hearing, Dotcom accepted the evidence of the US businessmen - including that donations were not discussed at the June 5 lunch. Instead, he said there was a second lunch - again on June 9 - at which the donations were discussed.

The interview with Dotcom was never disclosed to Banks' legal team before the Court of Appeal hearing. The newly disclosed material contradicts all the evidence given at trial by the Dotcom witnesses, wrote Mr Jones.

"It has never been part of the Crown case nor has there been any prior suggestion that there were two lunches within a matter of days of each other, at which both Mr and Mrs Banks were present," wrote Mr Jones.

"How the Crown can now properly pursue this prosecution in the circumstances is unknown ... the Crown case will accordingly have to be completely recast in a way which, with respect, is utterly untenable."