Rare Disorders NZ has lobbied for funding of Spinraza because clinical evidence showed it helped youngsters with type 1 SMA.
Foster said without the drug, young children with SMA were condemned to an early death.
"This means a child in New Zealand will die at an average age of 13 months when untreated, compared to those given the medicine at birth [for type 1], where just under 80 per cent show no clinical manifestations of the disease," Foster said.
"Are we forcing people with rare disorders to play a lottery game with odds never in their favour? In a country with a keen sense of fairness, is that acceptable?"
In February last year clinical experts deciding whether to fund the drug that would prolong the life of babies with SMA, said it would increase the burden of care if they lived longer - though not for loved ones.
A month later, a Pukekohe family launched a petition urging Pharmac to fund the drug which they believed would prolong the life of their then 2-year-old daughter.
Charlotte Bond was diagnosed with the degenerative genetic condition just before her first birthday.
SMA affects the control of voluntary muscle movement. It is caused by a loss of motor neurons.
If untreated, SMA is expected to make sufferers weaker over time, before causing an early death.
Spinraza is not a cure but has been proven to slow or stop the effects of the disease.
In severe cases the treatment can extend a child's life by years.
Pharmac director of operations Lisa Williams said the supplier of Spinraza, American drug company Biogen, submitted a revised funding application that was advanced to the agency's expert clinical advice committee for consideration in February.
Pharmac then included the drug in its prioritisation process but it did not have the budget to fund all the drugs on the priority list.
"We have determined that, at this time and given the amount of investment funding currently available, we will focus on progressing funding options that are ranked higher than nusinersen [Spinraza].
"Having a fixed budget means we need to make careful and considered funding choices in the interests of all New Zealanders."
Last year the estimated treatment retail cost of Spinraza was $1.4 million (US$750,000), now $1.16m because of exchange rates, though that figure would likely be discounted under a confidential rebate scheme.
It is then followed by four-monthly injections for the rest of the patient's life, worth about $125,000 per injection.
Pharmac said the pricing supplied by Biogen in its New Zealand funding application was confidential.
Foster said Rare Disorders NZ had repeatedly asked for a National Rare Disorder Framework to help address the issues.
RDNZ had set up a Parliamentary petition to urge the Government to acknowledge universal challenges faced by people living with a rare disease by committing to the development of a national framework.
About 300,000 New Zealanders live with a rare disease.
New Zealand lags behind most OECD countries in supporting people living with rare disorders and their families to access the best healthcare, the petition states.
"We believe a shift in mindset is needed for rare disorders to stop being considered in isolation, and instead to be regarded as a significant factor within health policy frameworks.
"This is in alignment with the global rare disease movement headed by Rare Disease International."
