He was confident the measures put in place around educating pupils and providing care meant the outbreak was under control.
Some of those affected had diarrhoea, he said, but most were being treated for vomiting, sore throats, runny noses and lower abdominal pain.
St Peter's is the largest boarding school in New Zealand on a single site. Of its 1150 pupils, 480 are boarders. It is a private, co-ed secondary school for years 7-13.
Burden said the cause of the outbreak was unknown, but it could have been brought to the school by boarders from Rotorua, where there had been a couple of cases of diarrhoeaand vomiting.
He said the outbreak represented a small percentage of the school role and although it was serious for affected pupils the school was operating normally.
The Ministry of Health had been advised and had provided advice to the school, Burden said.