But after Ford was acquitted, McIntyre appealed his conviction.

Today the Court of Appeal's Justice Forrie Miller, Justice Brendan Brown and Justice Denis Clifford released their decision.

They said for a charge of accessory after the fact to another crime to be successfully prosecuted the Crown must prove the substantive crime was committed - although it is not necessary that a person be convicted of that crime.

"In the circumstances, the Crown conceded that the successful self-defence claim by the alleged primary offender has the consequence that there was no murder," the Court of Appeal judges ruled.

"Accordingly, Mr McIntyre cannot be an accessory to a murder that did not occur.

"That is where a miscarriage of justice arises because, on the admitted facts, [McIntyre] could not in law have been convicted of the offence charged. Consequently, notwithstanding the guilty plea, we allow Mr McIntyre's appeal against his conviction on the charge of accessory after the fact to murder."

The Court of Appeal did not order a retrial when quashing McIntyre's conviction.

An explanation of the law: 'Appellant got lucky'

Professor Warren Brookbanks of the New Zealand Law Society Criminal Law Committee told the Herald that the facts of McIntyre's case were made the more interesting because of Ford's successful self-defence claim.

"Had the defence failed [Ford] would have been convicted of murder and McIntyre would have been guilty of being an accessory after the fact to murder," the AUT Law School professor said.

He added that cases such as McIntyre's do arise periodically in New Zealand's criminal courts.

Brookbanks also explained that an accessory after the fact is not a party to an offence.

"It refers to someone who assists an offender after the offence has been committed. It is a separate type of substantive crime. So liability is not for the crime committed but arises as an independent offence," he said.

There are four elements required to prove liability in a case of being an accessory, he said.

"The first element is that the accused must have offered comfort or assistance to someone who committed an offence and must have known of the facts that would have made that person's act an offence. Here, because the defence of self-defence was successful and because self-defence is a 'justification' at law, that meant, in effect, that no offence was committed.

"Because accessory after the fact liability is dependent on the commission of an offence by the principal offender, in those circumstances there was no crime to which accessory liability could attach," he said.

"In reality the appellant got lucky because no crime was committed by the principal."