In its first ruling of the year, the Yukon Court of Appeal has overturned a lower court conviction of a woman for second-degree murder, declaring the prosecution's strategy "impermissible” and prompting a re-trial.

The decision concerns Alicia Murphy, 34, who had appealed a jury's 2009 verdict that she drowned her acquaintance, Evangeline Billy, in the Yukon River along the Whitehorse waterfront in June 2008.

The appeal panel stated the Crown engaged in "oath-helping” and introduced inadmissible evidence involving two key witnesses that should not have been heard by the jury.

"What the witnesses said to police, their demeanour, emotional condition and co-operativeness should have had nothing to do with their testimony at trial, yet it was used to make the evidence more reliable,” wrote Justice Ian Donald on behalf of the appeal tribunal.

Oath-helping refers to bringing forward evidence "solely for the purpose of bolstering a witness's credibility,” according to a Supreme Court of Canada decision from 1987.

The term has roots in a "medieval practice” where an accused "could prove his innocence by providing a certain number of compurgators (references) to swear to the truth of his oath,” wrote then-Justice Beverley McLachlin — now Chief Justice — in a more recent Supreme Court ruling.

RCMP Const. Geoffrey Corbett testified that one of the witnesses was "co-operative and forthcoming” and that she was not intoxicated, with a demeanour that was "mostly calm but emotional at times,” Donald wrote in the decision, handed down Wednesday.

"All this would have left the jury with the impression that (the witness's) statements to the police were probably reliable and likely consistent with her testimony.”

Const. Craig Thur interviewed another witness on June 22, 2008, the day after the incident.

"(S)he was giving a story that was credible, with detail, believable .... she gave a pretty coherent and detailed statement,” he told the court at trial.

The defence did not object to the oath-helping evidence, the appeal panel said.

"The only evidence connecting the (Murphy) to the killing is the evidence of (the two witnesses). They were the Crown's case,” Donald said.

"Since there was no other overwhelming evidence of guilt, it cannot be said the verdict would necessarily have been the same despite the error.”

The witnesses' testimony may have had a "prejudicial effect” and "cannot be said to be harmless,” Donald added.

The Crown argued to no avail that the above police evidence was simply part of the "narrative” of the events that led to Billy's death.

The appeal court ruling overturns a Yukon Supreme Court decision that saw Murphy sentenced to 14 years in jail before she would become eligible for parole.

"Alicia Murphy has been convicted of a senseless and brutal murder,” Yukon Supreme Court Justice Ron Veale said in November 2009 of the crime, which saw Billy's beaten body dumped half-naked into the Yukon River.

Veale noted the violence of the incident, the supposed fact that Murphy had tried to make it look like a rape, and the three-day drug and alcohol binge which preceded the alleged murder.

The judge also noted Murphy had been on probation for an assault on her boyfriend at the time when Billy was killed.

Murphy's behaviour in jail since she was arrested in June 2008 also reflected badly on her, the judge said.

He referred to the fact she had been caught smuggling marijuana and tobacco into the Whitehorse Correctional Centre, bullying other inmates and threatening them with violence.

Veale noted that a second-degree murder conviction automatically comes with a life sentence.

Two witnesses, including her sister, testified that Murphy told them she killed Billy during an attempted robbery, then tried to make it look as if the woman had been raped.

Murphy and Billy were around the same age.

A date for the retrial has not yet been set.