Iridology is the the study of the iris - the coloured part of the eye - to help evaluate illness and weaknesses within the body.
Health sector lawyer Jonathan Coates said there was nothing to stop Mrs Nelson from continuing her practice because alternative therapists were not bound by any regulatory regime.
He said the case would reignite debate in medical circles about whether alternative therapists like iridologists and acupuncturists should be covered by the Health Practitioners Competence Assurance Act, which sets professional standards for doctors, nurses, midwives and other health professionals.
"If you did what this woman apparently did as a nurse, midwife or doctor then you might well find yourself being held to account by a professional disciplinary tribunal. There is none of that for the alternative therapists."
The Human Rights Tribunal could award damages but not strike her off, he said.
Mrs Nelson could not be contacted for comment.
New Zealand Medical Association chairman Paul Ockelford said regulating alternative therapists risked giving legitimacy to some "dubious" medical treatments.
"Iridology is one of a number of alternative health practitioners that do not have a scientific basis."
Dr Ockelford said he was concerned that Nelson could still practice.
"She was clearly totally out of her depth and went down a path that led to an avoidable and obviously very inappropriate outcome."
One of Mrs Nelson's former patients, David Nicols, told APNZ he had been sceptical of iridology when he first went to her Te Horo practice, but was now convinced by its benefits.
"She looked into my eyes and asked a few questions which were quite pertinent and put her finger on some of the things that were causing me to feel the way I was.
"She never professed in any way to be a doctor and certainly she never said that she would cure cancer, although she said that she could detect it in what she sees."
Mr Nicols said Mrs Nelson had discussed Mrs Main's condition with him in the past.
"Apparently she told her that she didn't think that she'd be able to cure anything but that she could try and help as much as she could, and that she would advise her to seek appropriate medical help. Whether that's true or not, that's what she told me."
Deputy Health and Disability Commissioner Tania Thomas this morning released a finding into the case, showing Mrs Nelson had failed in her duty to do no harm.
"The provider knew that she had exceeded the limits of her expertise and that the woman required advice from another practitioner, but she did not appropriately communicate this or discontinue her treatment of the woman, and she gave the woman information which accentuated the woman's fear of conventional treatment."
Ms Thomas said Mrs Nelson had also crossed professional boundaries by becoming close with Ms Maine - even going on holiday with her at one point during the treatment.
