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.