The public health watchdog says it's troubling that a health professional altered a prescription after a mistake but hasn't owned up to doing so.
In a report released today, Health and Disability Commissioner Anthony Hill said a three-year-old boy was given 10 times more painkiller than intended before a tonsillectomy two years ago.
Mr Hill said the boy received 85 milligrams of codeine, rather than 8.5 milligrams.
The boy was unharmed.
Mr Hill said an anaesthetist wrote the prescription in such a way that the decimal point between the 8 and the 5 was unclear.
Police document analysis revealed it was altered later to be correct.
Two unnamed nurses have been found to have breached patient rights for failing to check the dose.
Mr Hill said his office involved the police in trying to establish whether a prescription had been altered retrospectively.
He said the answer was yes, which was troubling, but he had been unable to find out who did it.
"It's most unfortunate that we were not able to find who it was. Most unfortunate that in that situation the person responsible did not feel able to come forward. All health professionals know that to correct notes in that way without identifying themselves is quite inappropriate."
Mr Hill said it was unusual for it to take the step of involving the police in a complaint over healthcare.