A cognitively impaired Indigenous man convicted of killing a 21-year-old man in Western Australia's north has won an appeal and walked free from jail after almost five years behind bars.

Gene Gibson, from the remote desert community of Kiwirrkurra, has been serving seven-and-a-half years' jail for fatally striking Josh Warneke from behind as he walked home from a night out in Broome in 2010.

A series of flawed police interviews more than two years later were deemed inadmissible, forcing prosecutors to drop a murder charge and accept Mr Gibson's guilty plea to manslaughter.

Mr Gibson's 2014 conviction was then appealed on the basis he suffered a miscarriage of justice because he did not have the cognitive ability or language skills to understand what was happening during the legal process.

Today the WA Court of Appeal made the unanimous decision to quash the conviction.

Mr Gibson, who appeared in court via video link with an interpreter, was released from Casuarina prison today.

Gene Gibson. (Supplied)

A passer-by found Josh Warneke's body by a Broome road in 2010. (Supplied)

WA Police Commissioner Karl O'Callaghan said police had already launched a fresh investigation into Mr Warneke's murder.

Mr O'Callaghan said he planned to meet Mr Gibson to personally apologise for his wrongful conviction.

Three out of 11 police who faced an internal review have formally advised that they did not accept the 2015 internal findings and made no expression of contrition.

Mr O'Callaghan said they would face disciplinary charges, with penalties ranging from fines to demotion.

Outside court, lawyer Michael Lundberg said the team was very grateful to the court for accommodating the appeal hearing "in a speedy fashion" and for quickly deciding to acquit Mr Gibson, who he described as being very happy.

Mr Lundberg also noted the detailed reviews undertaken by the Corruption and Crime Commission and the Internal Affairs Unit of WA Police.

"Both of those reviews helped focus the spotlight on Gene's case and his incarceration and they both provided the catalyst for the bringing of this appeal," he said.

Mr Lundberg said there were no winners in the case.

"He was 21 when he was arrested. He's now almost 26. He has been away from his country for a long time and is keen to return and be with his family," he said.

"There is also the reality that the life of another young man, Josh Warneke, came to an end in Broome seven years ago.

"Josh and Gene have never met but their lives are now intertwined."

Mr Lundberg also acknowledged Mr Warneke's mother, Ingrid Bishop, who has publicly supported Mr Gibson.

"(We) hope that in the future she obtains the justice she and her family deserve," he said.

Ms Bishop welcomed today's decision and said she hoped the investigation into her son's murder would be reopened.

"Today is a great day as it's the beginning of the next stage of Gene Gibson's life as a free man after nearly five years in jail," she said.

"I am so happy for Gene and his family.

"It's been a long haul for everyone involved and to see him walk free is one of the most profound moments in my life."

Mr Lundberg said it was too early to talk about whether Mr Gibson would seek compensation.

The reasons for the judges' decision will be published at a later date.