Doctors warn that waiting until symptoms appear - the only way it is diagnosed in New Zealand - means it is already too late for some children.
Three-year-old Zoey Butcher has just started dancing again.
But just a few months ago, she was losing the ability to walk.
Her mother Chauntel Wedlake had never heard of SMA - spinal muscular atrophy - before Zoey was diagnosed at two.
"She was struggling walking, she wasn't dancing any more, she wouldn't get up on her own at all. The change in just six to eight months, the decrease was incredible.
"It was scary," she said.
Wedlake could not understand why SMA was not included in the newborn screening programme.
"The difference it would have made if she was picked up from birth and then treated from birth would have been significant. She would have never declined, she would never have lost the motor neurones she's lost now, she could be running and jumping.
"And in a type 1 situation, it could save their lives."
About six New Zealand babies a year were diagnosed with the genetic disease, which destroyed spinal motor neurons, caused progressive paralysis and eventually affected their ability to speak, swallow and breathe.
Until very recently it had been a death sentence, with children with SMA type 1 (the most severe and the most common type) usually dead by their second birthdays.
Spinraza, which previously cost each patient $400,000 a year, has been fully funded for those 18 and under since January.
Risdiplam funding started in May.
Paediatric neurologist Gina O'Grady said clinical trials involving babies under six weeks of age have had "phenomenal results".
"At two years of age, 70 percent of those infants had no manifestations of SMA. They were typically developing, healthy children. And a small number had mild symptoms only. So it's life-changing."
Last year, the National Screening Advisory Committee considered O'Grady's application to add SMA to new-born metabolic screening, and agreed to progress it once treatment was available.
However since treatment has become available this year, nothing has happened.
By the time symptoms appeared - usually between four and six months when a baby fails to reach the usual milestones, like holding up their heads or rolling over - the damage was already irreversible.
"If you are so weak by the time you present because you've lost the majority of your motor nerves, treatment can't bring that back. That's why it's important to get in early."
Pharmac estimated between 30 and 50 children and young people with SMA types 1, 2 and 3 to be be eligible for treatment this year, with more in coming years.
Fiona Tolich, an adult with SMA who lobbied Pharmac and the government to fund Spinraza for children, said the Health Ministry needed to make screening "a priority".
Babies were declining needlessly due to a disease "that doesn't need to be terminal".
"What I find challenging is we fought so hard to get access to treatment. They actually have an obligation to do right by these babies," she said.
"That clock is ticking. There could have been a baby born today with SMA, because on average about five are born a year in New Zealand alone. This needs to be done now, this needs to be a priority.
"Forget paperwork, just get this in place."
Tolich, who moved to Sydney this year in order to get access treatment with Spinraza herself, said newborn screening there had already proven its worth in saving lives.
No-one from Te Whatu Ora was available for interview.
In a written statement to RNZ, Te Whatu Ora said it was "still considering options" for adding SMA to the Newborn Metabolic Screening Programme but no final decision had been made.
"Therefore a timeframe for implementation is not available at this time."
