"Most people with the bacteria do not get unwell. It is thought that about one person in a hundred with the bacteria will get meningococcal disease," he said.
Dr O'Connor said initial symptoms were similar to the flu, but the person could become unwell extremely quickly. The signs of meningitis include fever, vomiting, headache, sore neck and a rash.
"The main message to parents is to see a doctor if your son or daughter is becoming very unwell very quickly," Dr O'Connor said.
A spokesman for Wanganui Collegiate said the school's students were being offered support through school staff, counsellors and the chaplains.
He said boarders were told about the girl's condition on Thursday evening and day students on Friday morning.
Parents of students from the same year group as the girl were being updated on a regular basis.
The spokesman said some of the girl's closest associates at the school were among those who had been given anitbiotics as a precaution.
"We are in regular contact with Dr O'Connor and are being guided by his advice," he said.
The spokesman said the school was in "constant contact" with the girl's family.