About 2000 babies have been recalled to hospital to have their hearing checked after irregularities were found in screening checks for newborns.
Auckland, Hutt, Canterbury, Lakes, Bay of Plenty and Waitemata district health boards found irregularities with the Newborn Hearing Screening Programme.
Hawkes Bay and Waikato DHBs have also detected irregularities but are yet to report their results.
Associate Health Minister Jo Goodhew told Radio New Zealand's Nine to Noon programme that she does not know why the irregularities have occurred but is horrified by them.
"The screeners concerned have not done the process correctly and professionally.
"They have in some cases re-screened the same ear twice. In other cases they have screened one of their own ears instead of both of the baby's ears."
Ms Goodhew said that Auckland DHB noticed a problem with its programme in July, and the national screening unit then alerted all boards to look at their screening data, which shows when an adult's ear is being screened rather than a child's.
Ms Goodhew said 110 hearing screeners for newborns work at the country's 20 DHBs and as a result of the investigation, eight are no longer being used.
The minister said screeners do not have backgrounds as health professionals. They are given a two-week training course.
A report from the investigation is due in the New Year and Ms Goodhew said implementing its recommendations will be a priority.