Ahead of a major euthanasia legal battle, a local law professor and a US lawyer say there's a strong case the law here doesn't stop doctors helping mentally competent, terminally ill people to die.

Lecretia Seales' case in the High Court at Wellington starts on Monday.

Ms Seales, 42, is dying from a brain tumour. She says she has the right to end her life with medical help, instead of suffering a slow, painful, undignified death.

Ms Seales wants the High Court to clarify whether a doctor would be committing a crime if they helped her die.

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The University of Otago's Professor Andrew Geddis and Kathryn Tucker, from California's Disability Rights Legal Center, will publish an article on legal issues the case raises in the upcoming New Zealand Law Journal.

Prof Geddis and Ms Tucker said the law was currently vague, as Ms Seales' lawyer Andrew Butler has argued ahead of the trial.

"The lack of clarity in the law creates an uncertain legal environment making it very difficult for competent, terminally ill persons to die as they prefer: peacefully, on their own terms, with help from a medical professional, avoiding the final cruel bit of suffering caused by their illness," Prof Geddis and Ms Tucker wrote.

They said polls over the last 15 years showed most Kiwis believed patients suffering from terminal illnesses or unbearable pain should have access to aid in dying.

"Unfortunately, public support for providing a clear legal path for patients wishing to access aid in dying has not translated into parliamentary action," they added.

They said after earlier attempts in Parliament to pass "death with dignity" bills failed, Ms Seales had stepped into a legal "vacuum".

A case worth examining ahead of the Wellington trial was the Canadian case involving Kathleen Carter, the said.

Mrs Carter suffered from the degenerative condition spinal stenosis.

Ms Seales' case relies on provisions in the Bill of Rights Act enshrining the rights to not be deprived of life or subjected to cruel treatment.

In a similar way, the Carter case asked if Canada's criminal law was at odds with that country's bill of rights on the euthanasia issue.

Prof Geddis and Ms Tucker said the Seales trial would examine if the word "suicide" in local law could be given an alternative meaning, consistent with the Bill of Rights.

They said the Carter v Canada decision provided "extremely strong persuasive authority" an alternative meaning could indeed be made.

The Supreme Court of Canada found existing bans on euthanasia were incompatible with the country's Charter of Rights and Freedoms.

The Supreme Court also found the ban may have been counter-productive anyway.

"Here, the prohibition deprives some individuals of life, as it has the effect of forcing some individuals to take their own lives prematurely, for fear that they would be incapable of doing so when they reached the point where suffering was intolerable."

In response to the new paper, Care Alliance spokesman Matthew Jansen said the authors had largely ignored unsuccessful bids to overturn assisted suicide bans.

He said such cases included Fleming v Ireland, where a ban on assisted suicide was upheld.

Mr Jansen was wary of suggestions "alternative" definitions of suicide could be made.

"To try and differentiate between types of suicide is exactly the slippery slope that we should all be worried about."

Mr Jansen said Parliament had twice rejected pro-euthanasia bills at first reading so it would be "extraordinary" for a court to ignore that.

Care Alliance, the Human Rights Commission and the Voluntary Euthanasia Society have been cleared to provide evidence or have a say in the trial too.

Meanwhile, a British anti-euthanasia campaigner and author plans to visit New Zealand next week.

Dr Kevin Yuill believed changing the law on assisted suicide was the wrong decision for New Zealand.

"I hope that Lecretia Seales will be able to die the death she wants. I would have no great problem if she and her doctor came to a private agreement," he told the Euthanasia-Free NZ group.

"What I object to is the legal precedent set. It seems to be a way of avoiding the debate rather than having the discussion in parliament."