Solicitor-General determined to find how and why Alan Hall and Arthur Easton were let down by New Zealand's justice system

The Solicitor-General has called it a "significant miscarriage of justice". Now she's determined to find out why, and how, Alan Hall and Arthur Easton were so severely let down by the justice system.

Hall spent 19 years in prison for the murder of Easton, but on Wednesday, the Supreme Court quashed his conviction.

Hall said he's just living in the moment.

"Yesterday was yesterday, today's today," he told AM on Thursday.

On Wednesday, the Supreme Court said the criminal justice system failed him.

On Thursday, Hall's brother Geoff told AM he can't understand how critical evidence didn't make it to trial.

"We're told, 'Do you tell the truth, the whole truth, nothing but the truth', but we did.  It seemed the other side was holding critical evidence back that showed Alan could have never been the person that night that was running away, and that's not fair, not fair," he said.

Not fair, the Supreme Court found, was that the Police failed to disclose evidence, such as witness descriptions of the killer as Māori.

"It's one of the profound mysteries about Alan's case. That piece of evidence and how it was handled was so clearly wrong," said former Newshub journalist Mike Wesley-Smith.

Wesley-Smith started looking into the case six years ago and made the Grove Road Podcast alongside producer Maggie Wicks and editor Asher Bastion.

Wesley-Smith said why it took 36 years to quash Hall's conviction needs explanation.

The Police will review its original investigation and the Solicitor-General has launched an investigation into the miscarriage of justice.

"My focus now is on understanding how the Crown’s role in this miscarriage occurred and why the criminal justice system failed to remedy it earlier," said Una Jagose QC.

Attorney-General David Parker supports the investigation but couldn't say whether there could be consequences for the Crown lawyers.

"I suppose there could be but let's see whether they find fault in the meantime," he said.

"This case has revealed a lot of concerns, cracks and serious questions about how our country administers justice, and I think those questions deserve to be answered," said Wesley-Smith.

Hall will now have to apply for compensation, which could be in the millions. Anything he gets will be looked after in a Trust so that Hall, who is autistic, is protected.

If it was made into a legal drama, he knows just who he wants to play him: "Tom Cruise, because we're both the same age."

But the story is far from over for the family of Easton. Investigator Tim McKinnel believes the murder case should be reopened.

"There's still a surviving body of evidence from the case and there are lots of unanswered questions. There is no reason that this case can't be reinvestigated.  The Easton family deserve that," said McKinnel.

Because with Hall's conviction quashed, the question remains: who killed Easton?