The doctor of a lawyer fighting to be given the option of a doctor assisted death said her patient was competent to be able to make the decision to end her life.

A civil case being heard in the High Court in Wellington was brought by 42-year-old Lecretia Seales, who 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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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.

Ms Seales was present for most of this morning's hearing and was seated with supporters in the jury box.

Her lawyer Andrew Butler read an affidavit written by his client's doctor, who was given name suppression.

The doctor said that if the court clarified the law in Ms Seales' favour, she would be able to assist in Ms Seales' death if that was the decision she made.

She said when she had a discussion with Ms Seales about end of life options, her patient was not depressed or in a low mood - but was positive and keen to make the most of the time she had left.

"It is clear to me that Lecretia is competent and able to give consent."

She said if Ms Seales knew she had a choice about how her life would end, that would be a great comfort to her, provide her with a better quality of life and help her with coping with the challenges of her illness.

Ms Seales' oncologist, also with name suppression, said in an affidavit read by Dr Butler, the tumour was increasing pressure on her brain stem and it would exacerbate symptoms such as difficulty swallowing.

Ms Seales' life expectancy could be some weeks or a couple of months, the oncologist said.

Dr Butler 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.

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.

In an affidavit read by Dr Butler, Ms Seales said while it was "devastating" to be given the diagnosis that the cancer was incurable, she was determined not to fall apart.

"I resolved to live whatever remained of my life as well as I possibly could.

"I knew I wanted to really live while I'm still living," the affidavit said.

Ms Seales said she wanted to be in a position to ask a doctor to help end her life before she was unable to.

"I want to have a choice to be able to say enough is enough."

Ms Seales said she did not know what her death would be like, but it could be slow and difficult.

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

Today's case was heard in front of a packed public gallery.

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