A juror who sat on the retrial of a man previously convicted of murder said she had no doubt Nathan Williams was innocent of the crime.

The juror, who approached the Sunday Star-Times after reading about the case, criticised the police investigation into the 2008 death of teenager John Hapeta.

"It's just horrific he [Williams] could end up in prison for years," the juror said.

But police are refusing to accept they may have got it wrong, saying they are not looking for anyone else in relation to the killing.

Williams, 28, was found guilty in 2009 of the murder of 14-year-old John Hapeta in Weymouth, South Auckland.

He went to the Court of Appeal and won a retrial and then was found not guilty after he successfully argued he had been set up by Daniel Tumata, his co-accused in the first trial, who was found guilty of manslaughter for his part in the death.

Tumata told police he took a gun and accompanied Williams to Hapeta's family home in 2008 to do a "stand-over" of the young cannabis dealer.

He said he tried to stop Williams when he fatally struck Hapeta in the head with a hammer.

Williams maintained he had been set up and his defence speculated that Tumata and another man were responsible for the murder.

The juror said police appeared to have stopped investigating after Tumata pointed the finger at Williams.

"The Crown case was really weak from the start. There was just nothing in it."

Seeing Tumata testify confirmed her suspicions.

"There was nothing about him that was honest. He did anything to benefit himself."

Williams' testimony also swayed her.

"Initially I thought either he's an amazing actor or this is one fantastic set of circumstances."

Deliberations were quick, she said.

"He didn't do it. I know that."

That conclusion had a worrying corollary though: "The person who was holding the gun is still out there."

Williams' defence suggested a friend of Tumata's fitted the description of the second attacker but police have maintained they are not looking for anyone else in relation to Hapeta's death.

Detective Inspector Dave Lynch said there was "no evidence available to suggest that anyone else was involved" and police did not comment on private jury deliberations.

Lynch said police completed a "full and thorough" investigation into Hapeta's death but they would reassess if any new information came to light.

- © Fairfax NZ News