Security footage captured people on the school grounds near the dormitory just before the fire was lit. Police have been given the footage as part of their probe.
The school's board of trustees chairman Greg Moyle said the school surveillance equipment identified at least two people on the grounds, showing the arsonist was not working alone.
"We're hugely concerned someone could come from outside our school community on to our grounds and perpetrate this sort of act."
Today he reserved high praise for staff and pupils inside the burning building, saying thanks to the level-headed actions of those in the dormitory, a tragedy was averted.
"They acted incredibly professionally in how they dealt with the incident and as a board we're very appreciative of the staff there and proud of how the students reacted.
"This could have been horrific. As you can imagine, it's extremely concerning.
"We're incredibly lucky that it is a quiet time of the year and the boarding facility is not full. It could have been a lot more serious," he said.
"Anything with fire has huge potential for massive damage and loss of life. We take the safety of students very seriously and this is one case where the safety of students was threatened," headmaster Pat Drumm said.
He also praised staff and students for the way they handled the emergency, which had left the entire school community on edge.
"There was a mix of junior and senior students in the dormitory. The senior boys basically helped all the people in that dorm evacuate."
Drumm said damage was limited to one part of the building, as the flames were stopped by fire-rated doors.
"The dorms stood up brilliantly to what they needed to do at that time. There's some significant damage, but it could have been a lot worse," Drumm said.
Moyle said conditions were appalling when the blaze broke out.
"It was raining. To light a fire in the rain would be incredibly difficult," he said.
The school had given police security footage showing at least two people near the hostels around midnight before a fire fanned into flames at the rear of the building.
Since the attack, security and surveillance had been stepped up and there was a heightened awareness of people on the grounds of the central Auckland school.
Drumm said counselling and support had been given to affected students, some of whom had exams the following morning.
"Everyone was asleep and a number of the senior boys had NCEA exams the next day. To their credit, they all fronted and lost a bit of sleep but they've been looked after very well by NZQA."
He said parents of the 70 boarding students in the hostel at the time the blaze broke out were contacted soon after the incident and staff were addressed when they arrived at school.
"We got together with the group in the early hours and made sure messages were sent to their families, and the director of School House also sent messages.
"You can imagine a family many kilometres away, and it was lead on the news in the morning.
"I'm just grateful for the house master and the director who stepped up that evening and took the boys under their wings."
Drumm said he was not prepared to discuss the possible use of accelerant in the blaze.
"I haven't been informed of this by the police, I'd just be speculating and I don't want to do that, but things are going to be revealed in due course."
He said the school was now focused on getting the damaged dormitory repaired for next year's intake of boarders.
"We're fine at the moment but we need this dorm up and running, It needs repairs and we're looking at a lot of options of how we might house these students," Drumm said.
The headmaster said everyone was unsettled by the latest events.
"It's the year that keeps giving," Drumm said. "It's unnerving, but we need to find out what really happened here.
"The fire service and police are investigating it and taking things pretty seriously. We're sure we'll get to the bottom of this."
Fire and Emergency northern shift manager Daniel Nicholson said four fire trucks were sent to the Alberton Rd school after getting an emergency call at 12.09am on Wednesday.
He said the blaze was quickly extinguished but police and fire investigators were probing the cause and the fire at the rear of the building appeared to be deliberately lit.
"The fire is being treated as suspicious," Nicholson said earlier this week.
A Fire and Emergency NZ spokesperson said they were unable to comment while the investigation was underway.
A police spokesperson said officers were still working with fire investigators as part of the investigation and unable to comment further on specifics at this stage. Police were unable to release security footage for operational reasons.
A Ministry of Education spokesperson said help was on hand to support the school as needed.
