His son was supposed to be given 8.5mg of codeine before the operation, but he was given 85mg after both a nurse and senior nurse misread the prescription.
The nurses discussed that it was a large dose but neither checked with the anaesthetist.
Mr Dagg told Radio New Zealand that when the nurse gave his son the codeine, his wife commented: "That's an awful large syringe."
When the nurse returned to give codeine to his daughter, the dose was "an awful lot smaller".
"This was when alarm bells sort of went off. The nurse went and fetched the surgeon, who immediately took control of the situation and said, 'Get him into theatre now, we're going to pump his stomach and continue with the operation'.
"My wife was pretty distraught at this stage. The thoughts that were going through her head were things like, 'Is this the last time I'm going to see my son?"'
Mr Dagg said it was a "stressful" situation.
"Even now my wife can't forgive herself for not stopping the nurse with that huge, big syringe."
Mr Dagg said the nurses should have realised they were giving a child a dose "way outside the normal operating spectrum".
"Professionals who do the job day in, day out should realise that, 'hang on, there's a mistake here, we need to clarify this'."
He said one could argue about the decimal point in the dose, which the nurses had not seen, but "10 times is a major error".
Mr Dagg said because the operation took place in a private hospital, he was billed for the cost of pumping his son's stomach.
His son was now fine and there were no lasting health effects.
"But there's always that 'what if?' in the back of our minds."
Mr Dagg was not at all satisfied with the commissioner's report, describing it as a "pretty average outcome for what could have killed my son".
The commissioner found the nurse demonstrated "very poor judgement" and the actions of the senior nurse were "concerning". Both had breached health codes for failing to provide services to the boy with appropriate care and skill.
The report also criticised the legibility and comprehensiveness of the anaesthetist's documentation.
The commissioner recommended the nurses and the hospital provide written apologies to the boy and his family and asked the Nursing Council to consider reviewing the competence of the nurse.
The hospital had also been asked to undertake staff training on the importance of clear, open and supportive communication with patients and their families.
