Pharmac is being accused of imposing a death sentence on some patients by refusing to approve funding for an expensive medicine.
The Organisation for Rare Disorders, Lysosomal Diseases New Zealand and the Muscular Dystrophy Association launched a campaign on Tuesday for better access to medicines for those with rare diseases.
Lysosomal diseases are caused by an enzyme deficiency, and the medicines used to treat them - enzyme replacement therapy (ERT) -can cost $500,000 per patient per year.
Masterton mother-of-three Allyson Lock told Nine to Noon she is one of four people in New Zealand with Pompe, a neuromuscular disease.
Pharmac says it will not fund treatment costing $500,000 per year for her alone.
Ms Lock said the Government's drug-buying agency has told her it has the money to fund the drug for the first year, but not necessarily for longer.
"They were worried that it would blow the budget for next year so they declined me, at the same time admitting that there was no other medication and that this medication would help me.
"So, effectively, they've given me a death sentence - because without the medication, I will die."
The head of the Organisation for Rare Disorders, John Forman, says a major drug supplier was set to do a deal, but it fizzled.
"We understand that there would be a significant discount on the total costs of treating all patients, so that the average cost per patient would come down considerably and that's an offer which is apparently on the table, but which had been rejected by Pharmac.
Pharmac medical director Peter Moodie says the treatment sought by Mrs Locke was not only expensive, but there was a lack of evidence it would work.
"When you're going to spend a lot of money, it does focus your mind and it makes you ask the question 'What is the evidence that this drug will do good'".
Pharmac says the diseases are tragic and there may not always be a magic bullet to fix them.
However, an Auckland doctor says medicines used to treat rare, genetically inherited diseases certainly work, but are costly.
Metabolic paediatrician Callum Wilson says there are about 40 of the diseases and enzyme replacement therapy works for about 10.
Dr Wilson says this sort of therapy can be very effective, but is not always.
"The literature suggests that in the short to medium term, they work quite well - very well in some patients - we just haven't got long-term (10 to 20-year) data on them.
"But they main reason they're not being funded is because they're too expensive - not because they don't work."