A coalition opposed to legalising euthanasia has welcomed the latest legal development in the case of terminally-ill Lecretia Seales.

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.

She wants the High Court in Wellington to clarify whether a doctor would be committing a crime if they helped her die.

Care Alliance was one of three groups that wanted a say in the trial.

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Her case, to be heard at a trial later this month, would have wide and profound consequences for disabled people, the Care Alliance says.

But Ms Seales' lawyer has said the case was "discreet and narrow" and would not cause the snowball effect suggested.

Late last month, Justice David Collins granted the alliance, the Human Rights Commission and Voluntary Euthanasia Society a say, within tight guidelines.

But a misunderstanding caused confusion about what evidence the alliance, which included Family First, Hospice New Zealand, and the New Zealand Health Professionals Alliance, could provide.

Justice Collins announced yesterday evening Care Alliance could now provide evidence relating to the possible impact of the case on disabled people.

Care Alliance spokesman Matthew Jansen said he was pleased with the judge's clarification.

"It certainly gives us clarity on what we're allowed to provide evidence on but also that we can provide submissions on the whole case, and all of the issues that are raised," he said this morning.

Mr Jansen said the case raised wider issues about suicide and the value of human life.

"Disabled people have many challenges in their lives already. And if it is possible for a person to say 'my life is so difficult that it is not worth living' that is implicitly a challenge to disabled people who live with difficulties every single day."

Mr Jansen said the case had potential impacts for young people with mental illnesses, and for the elderly.

Mr Jansen said Care Alliance had to submit its evidence by 5pm tomorrow and would be given access to evidence Ms Seales had filed.

Yesterday evening, Justice Collins also said Care Alliance and other interveners could now access evidence Ms Seales and her legal team provided the court.

But only redacted versions of the evidence would be given to interveners, following a confidentiality agreement lawyers for Ms Seales and the Attorney-General had reached.

In a judgement on April 21, Justice Collins said Ms Seales' general practitioner, whose name is suppressed, respected and understood Ms Seales' wishes.

But the doctor was only willing to help Ms Seales die if the court decided doing so would not be a crime.

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

However, Dr Butler previously told the court "we don't know what the law is" and Ms Seales' case was testing the law.

The court heard Ms Seales was concerned not just for herself, but about the impact her circumstances would have on her family.

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

Ms Seales' lawyers did not immediately return messages seeking comment this morning.