The woman fighting to have the option of dying on her own terms says she was not coerced into making the decision to choose whether to die before a terminal brain tumour kills her.

Lecretia Seales, 42, has possibly only months to live, after being diagnosed with incurable brain cancer.

She has taken a legal battle to the High Court at Wellington to have the law clarified over whether her doctor can assist in her death before her quality of life becomes intolerable and undignified.

Her case, in front of Justice David Collins, relies on provisions in the Bill of Rights Act enshrining the rights to not be deprived of life or subjected to cruel treatment.

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The former lawyer was present periodically throughout the day, sitting in a wheelchair next to her mother, often with her eyes closed while she listened to her lawyers' submissions.

Five counsel made up Ms Seales' legal team, and two of those lawyers spent the day giving submissions on their client's personal situation, similar legal cases and how the Crown's case contradicted the Bill of Rights Act.

One of her lawyers, Chris Curran, said Ms Seales' dignity was a central part of her claim.

That dignity underpinned the Bill of Rights framework, he said.

It was also dignity that was being "assaulted" by the Crowns case, he said.

Another of Ms Seales' lawyers, Andrew Butler, told the court that his client, in an affidavit, had sworn she had not been talked into her euthanasia decision and she wanted the choice to be able to say when enough was enough.

Her family, sometimes reluctantly, had accepted her decision, she said.

It was a "sound decision" that she trusted herself to make, Ms Seales said.

He read an affidavit written by his client's doctor, who was given name suppression, who said she would be prepared to assist in Ms Seales' death if that action was ruled legal.

She said it was clear that Ms Seales was competent and able to give consent.

Dr Butler also read a number of affidavits from international palliative care experts, that said end of life care did not address all suffering, including the psychological impacts of suffering.

Surgical oncologist at the University of New Mexico, Katherine Morris, said in an affidavit that for some terminally ill patients having lethal medicine prescribed gave them "reasons to live" and made their last months less stressful.

American doctor Eric Kress, involved in palliative care in Montana, said some people faced a "hard death" regardless of their care.

Ms Seales had been living with the diagnosis since March 2011, when a neurologist determined brain cancer after she had been experiencing headaches and loss of vision in her left eye.

Also joining Ms Seales' case are the Care Alliance and the Voluntary Euthanasia Society.

The public gallery was packed for today's hearing.

About 10 of Ms Seales' supporters, including her parents and high-profile lawyer Sir Geoffrey Palmer, were seated in the jury box.

The hearing is expected to last until Wednesday.