Up to 2000 babies have been recalled to hospital to have their hearing checked after the Ministry of Health discovered "irregularities" in its national newborn screening programme.
One of the babies recalled, a 10-month-old boy, was found to have a congenital hearing defect.
Officials would not say what the irregularities were, but it is believed the hearing of some newborn babies was not checked.
The Herald understands some screeners from the six affected district health boards were testing themselves instead of the babies.
Associate Health Minister Jo Goodhew said she was "very concerned" when she was told in July of the situation.
Asked if any workers had been dismissed or stood down, she said: "I am aware a number of screeners no longer work at the DHBs."
Two boards told the Health Ministry five months ago they had concerns about their newborn hearing screening programmes.
National Health Board national services purchasing director Jill Lane said that as a result the Ministry initiated a review of all DHB newborn screening activity.
Six boards - including Auckland, Waitemata, Bay of Plenty, Lakes (Taupo), Hutt Valley and Canterbury - were found to have irregular screening results, and two, Waikato and Hawkes Bay, are still providing information.
It is not known how long the issues had been going on or how old some of the babies are now.
Ms Lane said a report into the investigation was completed this week, but its recommendations would not be made public until it was sent to affected families early in the New Year.
Ms Lane did not know how many of the 2000 families sent recall letters had responded, but said that of those that had, the 10-month-old was the only delayed diagnosis.
Mrs Goodhew said while that family would be disappointed, they were now getting the support they needed at birth.
"I understand there are no waiting lists for cochlear implants. Children have priority so that child will have cochlear implants if that's what he needs."
The programme is intended to check the hearing of the 60,000 babies born each year.
All babies are supposed to be screened within a month, to complete an audiology assessment in three months and start any required treatment by six months.
"We want to pick up any irregularities in a child's ability to hear as early as possible," Mrs Goodhew said.
About 2000 babies a year need further hearing assessment, and the programme expects to find one baby in 1000 with congenital hearing loss.
Mrs Goodhew said she would not pre-empt the report by giving details.
"It's certainly not what you want. You want a standard of screening and you want everyone to conform to that standard and in this case it hasn't happened."
She said she had more confidence in the programme now a thorough investigation had been made, and she was comfortable with the "comprehensive recommendations".
The country had about 115 technically trained screeners.
Specialist: Early diagnosis vital
Delays in diagnosis of moderate to profound hearing loss can cause lifelong complications for children.
Auckland cochlear implant surgeon and ear, nose and throat specialist Michel Neeff said the earlier hearing loss was picked up the sooner it could be treated.
"When a child is really young the brain is able to adapt and develop in a way to understand speech and develop language," he said.
"And if you miss that critical period, which is up to the age of three or four, there can be long-term speech and language problems."
Missing mild hearing loss was a minor issue but undetected profound hearing loss could result in a child never learning to speak.
Mr Neeff said many children with congenital hearing loss had cochlear implants to aid hearing, by the time they were six months old.
A child tested at 12 months could easily catch up on language milestones, but if the testing were left until age four or five, the consequences could be disastrous.
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