Allowing other parties to weigh in on a terminally ill woman Lecretia Seales' euthanasia bid would cause intolerable and unfair delays to an upcoming trial, her lawyer says.

Ms Seales, 42, is dying from brain cancer and said she had a right to end her life with medical assistance before her suffering became unendurable.

In a hearing in the High Court at Wellington today, three groups sought to have their say on whether Ms Seales could take a lethal dose of drugs.

The Human Rights Commission, Care Alliance Trust and Voluntary Euthanasia Society want to participate in Seales' trial, which starts next month.

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Ms Seales' lawyer Dr Andrew Butler this afternoon opposed applications from these other parties to join the legal challenge.

Care Alliance, a coalition opposed to euthanasia and assisted suicide, said groups that worked with the elderly, infirm and in the field of suicide prevention had to be heard.

The coalition's lawyer Victoria Casey said Care Alliance had special expertise and would present evidence the courts could benefit from.

She said changing the Crimes Act to allow Ms Seales' euthanasia would impact "the whole of society".

"There is harm to the vulnerable in society by lifting the ban," she told Justice David Collins. "These are not lightweight matters."

Ms Casey said allowing Ms Seales to be administered a lethal dose of drugs would also create "indirect or subtle pressure" on other people.

She said overseas evidence showed "the concern about being a burden" was a major reason for older people exploring euthanasia as an option.

Ms Casey said respected medical associations around the world regarded euthanasia as unethical.

She said it was wrong to suggest the Attorney-General alone could tackle the case without input from special interest groups.

Ms Casey said overseas, many people lived "every day" with the kind of suffering Ms Seales described.

Kate Davenport QC, the Voluntary Euthanasia Society's lawyer, said Ms Seales' legal challenge would not create "a personal decision in isolation".

Instead, she said the issue was one of public policy and would "affect the rights and obligations of many in the future".

Ms Davenport said the Attorney-General already acknowledged he could not provide all the necessary evidence for the case.

She said the Society did not advocate euthanasia for the disabled, or for people unable to make decisions for themselves.

But Ms Davenport said other New Zealanders were in a similar position to Ms Seales, and the Society could play a useful valuable role in discussions about the "wider public importance" the case raised.

Human Rights Commission lawyer Matthew Palmer QC said the Commission could play a helpful role as a neutral, independent party assisting the court.

Dr Palmer also said the outcome of Ms Seales' bid would not be "confined" but would have impacts on other people.

"If ever there was a case of widespread public importance, this is it."

He said the Commission did not take a view on the morality of euthanasia but had helped courts navigate tricky public policy issues before.

"The Commission recognises that the proceedings are sensitive and of intense personal concern to the plaintiff."

The Commission had "independent expertise" on human rights relevant to the euthanasia debate and was seeking to file written submissions, but not evidence, Dr Palmer said.

For the Crown, Professor Paul Rishworth said the Seales case raised debates about palliative care, ethics, euthanasia in other countries, and definitions of culpable homicide.

Prof Rishworth said the Crown could not exclude the possibility other parties could help the court with Ms Seales' case.

It is illegal under the Crimes Act to incite, encourage, aid or abet anybody to commit suicide.

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

Ms Seales' lawyer Dr Andrew Butler said next month's trial did not need mountains of "extraneous" evidence from other parties.

"This is our case. Not theirs."

He said it was strange to argue the other parties needed to help the Crown, as the Crown was already "the expert" in criminal law and bill of rights issues.

Dr Butler said with the trial set to start on May 25, there was "no luxury of time to co-ordinate evidence between parties."

He said this meant intervening parties would come to the trial with double-ups in evidence, spawning additional costs and time delays.

"There's a real risk here of the case ballooning out."

Dr Butler said Ms Seales' case was "discreet and narrow" and would not trigger the snowball effect some other parties suggested.

"We've heard about the fears that old people are going to be bumped off," Dr Butler said. "We're not dealing with somebody who's elderly. We're not dealing with somebody who's disabled."

He said the case was fundamentally about clarification of criminal law.

"We don't know what the law is. It's never been tested. This case is testing the law."

Although he said the case was "novel" in New Zealand, similar issues were discussed in high-profile cases abroad.

Dr Butler said these cases, from Ireland, Canada, the UK and USA, already provided material a local court could find useful, so input from intervening parties was superfluous.

Justice Collins reserved his decision this afternoon but said given the obvious time pressures Dr Butler and Ms Seales were facing, he would make his decision as soon as possible.