Christchurch woman Helen Milner, dubbed the "Black Widow", will head to the Court of Appeal today to begin her fight for her dead husband's body parts.

Milner was convicted of the murder of her second husband Phil Nisbet by poisoning him with crushed sedatives in his food in 2009.

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She was sentenced to life in prison with a minimum non-parole period of 17 years.

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Milner, 55, has so far unsuccessfully appealed against her murder conviction.

Nisbet's sister, Lee-Anne Cartier who helped secure Milner's conviction, has labelled her now decade-long court battle "insane".

"It's insane to think that this long it's still going on," she told Newshub. "It's insane to think after 10-and-a-half years. It's not fair."

Her brother's death was originally ruled a suicide.

But Coroner Sue Johnson raised suspicions with Nisbet's death which then led to police reviewing the case and ultimately launching a homicide investigation.

The Court of Appeal earlier said they had no jurisdiction over the samples.

Brother and sister Philip Nisbet and Lee-Anne Cartier. Nisbet was murdered by wife Helen Milner. Photo / File
Brother and sister Philip Nisbet and Lee-Anne Cartier. Nisbet was murdered by wife Helen Milner. Photo / File

However, in a High Court judgment released in July, Justice David Gendall ruled the Court of Appeal does in fact have "custody" of the samples and must now decide whether they are to be released to Milner's lawyers.

"The proper course now, as I see it, is for the Court of Appeal to reconsider the matter," Justice Gendall concluded.

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Milner is trying to secure the release of Christchurch truck driver Nisbet's hair, blood, urine, and liver samples, which she says will prove her innocence.

She hopes that the test results may support an application to the Governor-General for an exercise of the Royal prerogative of mercy, which provides a special avenue for criminal cases to be reopened "where a person may have been wrongly convicted or sentenced".

The samples are held by the Institute of Environmental Science and Research (ESR).

The Nisbet family, from left, sister Lee-Anne Cartier, brother Andrew Nisbet, parents Jim and Yvonne Nisbet and brother Roger Nisbet. Photo / Supplied
The Nisbet family, from left, sister Lee-Anne Cartier, brother Andrew Nisbet, parents Jim and Yvonne Nisbet and brother Roger Nisbet. Photo / Supplied

Dr Martin Sage, the forensic pathologist who performed the autopsy on Nisbet, found that the "ingestion of excessive promethazine was the operating cause of death". His conclusions relied on samples taken from Nisbet's body.

Milner argued in the Court of Appeal that the jury's verdict was unreasonable because the prosecution did not prove beyond reasonable doubt that it was possible for her to have administered the drug Phenergan without her husband noticing its bitter taste.

The court found it was not necessary for the prosecution to prove exactly how the mother of two had administered the drug so Nisbet did not know about it.