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 said.
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, Ms Davenport said the issue was one of public policy and would "affect the rights and obligations of many in the future".
She said the Attorney-General already acknowledged he could not provide all the necessary evidence for the case.
Ms Davenport said the upcoming hearing would likely have to involve careful interpretation of the definitions of assisted suicide and culpable homicide.
She said the society did not advocate euthanasia for the disabled, or for people unable to make decisions for themselves.
But she said there were other New Zealanders in a similar position to Ms Seales, and the society could play a valuable role in contributing to 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.
He said the commission did not take a view on the morality of euthanasia but had helped courts navigate 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.
Ms Seales' lawyer Dr Andrew Butler is opposing applications from other parties to join the legal challenge.
It is currently 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.
A Human Rights Commission spokeswoman earlier told the New Zealand Herald 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."
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.
He said the commission did not take a view on the morality of euthanasia but had helped courts navigate 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.
