A transgender woman found dead in a car in Wellington last year may have died from an overdose rather than being strangled to death.

Paddy Jonathan Woods is now "free at last from the shadow of the accusation" of killing his girlfriend, Zenith (Zena) Campbell, after a High Court judge threw out a murder charge against him on the day his trial was due to start.

Woods was accused of murdering 21-year-old Campbell after attending a party with her in February last year.

Campbell, who wanted to be known as Zena, was found dead in a car in Aro Valley on February 11, 2018.

In his judgement released this afternoon, Justice David Collins said Woods had told police he put Campbell in a choke hold as part of their consensual sexual activity a couple of nights before the death.

But new evidence revealed it's possible the death was related to a lethal combination of drugs and alcohol in Campbell's system, as opposed to any further choking that could have occurred immediately before her death.

Zena Campbell sent this picture of herself to her father shortly before she was murdered in Wellington last year. Photo/Mark Mitchell

Forensic pathologist Amy Spark was called to court for a last-minute pre-trial hearing on the matter this morning, where Justice Collins asked her to clarify her opinion on whether any "consensual" strangling was likely to have killed Campbell.

Spark said there had always been a "degree of uncertainty", and her interpretation of the situation had been slightly altered after toxicology findings came back.

She did not believe the level of choking or strangulation that may have occurred immediately prior to Campbell's death would be severe or prolonged enough to be "substantial or operative" in causing the death.

Asphyxial signs were normally present after such a form of strangulation if it was prolonged enough to cause death, and Campbell did not show those signs, she said in her report.

Spark's post-mortem findings showed Campbell had methadone, oxazepam, Ritalin, alcohol and cannabis in her blood.

Campbell, who was on the methadone programme, had been warned just a couple of weeks prior that the use of methadone with other substances created a higher risk of overdose.

Campbell was described by a friend as "next level wasted" and incoherent in the hours before her death.

After reflecting on Spark's evidence for a few hours, Justice Collins announced he was dismissing the charges against Woods.

He said he was satisfied a jury could not reasonably find Woods guilty of murder or manslaughter.

Speaking outside court, Woods' lawyer Kevin Preston told reporters it was a year and a day since Campbell had died "in tragic circumstances".

Woods consistently maintained throughout that period that he was not responsible for the death of his partner, Preston said.

It has been a "traumatic, trying time for Mr Woods and, of course, for the family and friends of Zenith Campbell", he said.

"Mr Woods, as a consequence of the accusation, lost his liberty for a time, lost his job and additionally he was accused of being responsible for Zenith's death [and underwent] intense scrutiny of his personal life."

"Mr Woods wishes now to try rebuild his life and to grieve for the passing of his partner, free at last from the shadow of the accusation," Preston said.

Zena's father, Chris Campbell, said outside court they could only hope Zena "passed quietly".

"Even if [Woods] had been convicted or anything like that, it wouldn't have brought her back," he said.

"There's a lot of contributing factors that made the perfect storm."

Uncle Craig Campbell said no one person was responsible for Zena's death, and that it was "a collection of things that had gone wrong".

Carol Campbell had a close bond with Zena Campbell. Photo/Mark Mitchell

She had struggled with addiction, but had begun a methadone programme two weeks before her death.

Campbell, who Zena lived with for more than five years as a child, said she had only ever known her as a boy, one she described as a "lovable ratbag" with a troubled past.

"He overcame a lot. His life was a struggle. It was a struggle, especially, with ADHD. But he muddled through.

"He had a struggle with drugs, with injecting. He had just decided 'that's it, I'm going to get clean', and he got on the methadone programme and two weeks later he was dead.

"He was starting to become happy, starting to get his life together."

A few months before her death, Zena spoke to her aunt and explained she had decided to start living as a woman and taking hormones.

"I said 'Zen, we love you unconditionally'."

Zenith felt like Campbell's own child - the pair have always been close.

"I was the first one who ever held him when he was born ... he was mine, he felt like mine."

Zenith was dyslexic and had ADHD, so didn't find school particularly easy, but was always the type of person to stand up for the underdog, Campbell said.

"He always had a very kind heart. He was the apple of all our eyes."