Akuhata was bashed by a "fight club" gang member drunk on moonshine while on remand at Mt Eden prison for Edwards' murder, leaving him with a brain injury.
He was declared fit to plead by the Court of Appeal before admitting the charge of killing Edwards.
Akuhata had four previous convictions for male assaults female, three of which involved Edwards.
Akuhata and Edwards had been in an "on again, off again" relationship for about six and a half years.
But when Akuhata murdered her, they were apart and there was a protection order in place for Edwards and her two daughters.
One night in July 2012, Edwards agreed to go to an event at a bar with Akuhata. During the night, he became angry at Edwards about text messages she had received.
According to facts accepted at the time, they left the bar about 1.55pm and were arguing on the Lower Tarewa Rd bridge when he lifted her up and over the railing into Waiarohia Stream.
He then grabbed her hair and held her head under the water until she stopped showing signs of life.
He went home where he told a family member Edwards was dead.
His appeal last year focused on numerous points, including an argument his previous lawyer, Catherine Cull, did not handle his case correctly, given his brain injury.
Human rights lawyer Tony Ellis questioned whether there had been some pressure on "a brain injured person" to plead guilty.
But Cull said she had several discussions with Akuhata about the guilty plea and he was "keen" to enter it. She said she gave him plenty of time to think about it first.
But in their written decision, released today, the Court of Appeal judges said no criticism could be made out against Cull or the court process.
"Mr Akuhata should now be satisfied that the criminal justice system has served him well and that he rightly faces the sentence imposed on him," the decision said.
"What has happened emerges with tolerable clarity from the mass of material placed before us. In a fit of jealous rage Mr Akuhata threw or pushed Ms Edwards over the parapet of the bridge into the stream below. He then climbed down or jumped into the stream and held Ms Edwards' head under the water until she was lifeless.
"He made his way home, soaking wet and very upset, and admitted what he had done to his cousin, his friend and his neighbour. He admitted it again to a fellow inmate in prison the next day. But he declined to say anything to the police.
"As Mr Akuhata recovered from the traumatic head injury inflicted on him in prison four months after he murdered Ms Edwards, Mr Akuhata began struggling, increasingly, with the enormity of what he had done and its consequences ... after pleading guilty, Mr Akuhata knew he had done the right thing. He said as much to the probation officer."
Now, confronted with the "bleak prospect of 15 years in prison", Akuhata had changed his mind, the judges said.
"He has compared his sentence with those imposed on another or other prisoners convicted of manslaughter. This has led him again to try and convince himself that he killed Ms Edwards accidentally."
The court dismissed his appeal.
