A test which could have prevented hospital staff - who were checking the hearing of thousands of newborn babies - from screening their own ears was not used because it cost too much.
A Ministry of Health report into the scandal, which led to 2064 children being recalled for further screening, has recommended an alternative test be used.
The situation, which led to a delayed diagnosis of profound hearing loss for at least one child after the 10-month-old was recalled, was first reported by the Herald late last year when some screeners from the six affected district health boards were revealed to be testing themselves instead of the babies.
Yesterday, the ministry released a report into the screening irregularities at six district health boards where two screeners were found to have tested their own ears instead of the babies' ears.
A total of eight screeners had irregular results. Other problems with the newborn screening programme included testing the same ear twice on a baby, and screening one ear of the baby and one of their own ears.
The programme was introduced nationwide in 2010 to catch hearing loss in children as early as possible.
The report, which was sent to the parents of babies affected early in the new year, recommended DHBs change the screen to the auditory brainstem response test (AABR) because it meant screeners could not test their own ears.
It said despite the test being used in other programmes it was assessed as "not cost-effective" at the time the programme was established. But a source within the sector said the cost was now far greater.
"If that was an important quality factor before and they've gone cost over quality, and they've done the same thing not setting up a national database and getting the DHBs to set up 20 DHB databases, what cost is all of this to us?"
Changing to the AABR screen would mean new equipment and training worth hundreds of thousands of dollars, the source said.
That was on top of the potential personal cost to children whose congenital hearing defects were missed, which could lead to delayed treatment and speech and language development.
Ministry spokeswoman Marian Mortensen said there had been a wide range of programme and individual performance checks but they all failed to detect what had been going on.
"Markers for identifying screening adult ears had not been identified, as the practice was not anticipated."
Since then, all DHBs had reviewed all their programmes.
Associate Health Minister Jo Goodhew said she had been concerned to learn of the irregularities that had been found in the screening process at some DHBs.
Less than perfect
2064 children recalled for hearing checks
2 people paid to undertake checks tested their own ears
8 screeners were found to have irregular results
Additional reporting by Vaimoana Tapaleao