"Under this new assessment, if there is a patient who doesn't meet the normal criteria, their doctor can apply to have the patient put on it."
Pharmac acting chief executive Steffan Crausaz said the agency saw the change as an improvement based on individual need rather than rarity of a condition.
Funding would increase from the current $4 million to about $8 million over the first year, and each case would be assessed by expert clinicians.
"This means careful decisions will have to be made about which medicines are affordable," he said.
But people with rare disorders say the change will mean they will face more competition in getting funding for drugs.
NZ Organisation for Rare Disorders executive director John Forman said he acknowledged there were some benefits in the change, but not for his members.
"Essentially they are making a few improvements because there were some complications related to Exceptional Circumstances," he said.
"But they haven't done anything towards funding special medicines; in fact, it makes it more difficult."