The secretary then called the police, who arrived "straight away" Doherty said.
There was so far no indication the man was related to any of the Browns Bay pupils.
Police had not indicated the call was linked to yesterday's bomb threat either, she said.
Police confirmed they were aware of what had happened and were investigating, but would not say more on Friday afternoon.
While police were called an alarm sounded telling children the school was in lockdown and they needed to return to their classrooms and shut the door.
Doherty said all children were indoors in minutes and she was pleased with her staff's response and the way the school handled the situation.
While children and staff were inside, some under desks, four armed policemen did a thorough search of the grounds and bush.
Police trying to trace call and it was so far unclear who made it.
"At this point in time there is no indication it is anybody linked to the school," Doherty said.
Today there had been a few nervous children arriving through the gates, but staff had talked through what happened in assembly and how to keep safe.