The Human Rights Commission and two other groups want to join a legal challenge by a terminally ill woman seeking the right for a doctor to help her die without criminal prosecution.

Lecretia Seales, 41, is dying from brain cancer and believes it's a "fundamental human right" to be able to choose to end her life with medical assistance, if she wants to, before her suffering becomes intolerable.

In a legal first in New Zealand, the senior legal and policy adviser at the Law Commission filed a statement of claim in the High Court seeking a ruling to determine whether her GP could lawfully administer a lethal dose of drugs.

Assisting suicide is a crime punishable by up to 14 years in prison but Ms Seales' case relies on the provisions in the New Zealand Bill of Rights Act that protect the rights to not be deprived of life or subjected to cruel treatment.

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If successful, the bid would allow the doctor to euthanise Ms Seales because of her specific circumstances and would not set a precedent. But a favourable High Court ruling would allow others to follow suit and potentially send a signal to Parliament for further law reform.

The case, Seales v the Attorney-General, has been set down for a one-week hearing in the High Court at Wellington next month.

But three other parties - the Human Rights Commission, the Voluntary Euthanasia Society of New Zealand and the Care Alliance Trust - are seeking to join the case and have their say in court.

Voluntary Euthanasia supports Ms Seales' position, while the Care Alliance is opposed to assisted suicide and euthanasia.

Asked whether the Human Rights Commission supported or opposed Ms Seales' bid, a spokeswoman said the case raised important human rights issues with implications for many people.

"We believe it is important the court has the benefit of an independent perspective on human rights principles and the legal framework which applies in cases like this."

A hearing in the High Court at Wellington today will determine whether the other parties can join.

Ms Seales' lawyer, Andrew Butler, confirmed he would oppose the applications, while a spokeswoman said the Crown did not.

At the hearing next month, the Attorney-General will be represented by two Queen's Counsel, Solicitor-General Mike Heron and Paul Rishworth.

Mr Heron has a background in medico-legal hearings, having previously acted on behalf of medical disciplinary tribunals, while Mr Rishworth is an expert in human rights and constitutional law which he taught at the University of Auckland.

The judge presiding over the hearing, Justice David Collins, is considered an expert on medico-legal matters.