He said so far it looked like two classrooms in the school's senior block had been gutted by the blaze - the home economics and social sciences rooms.
"But it was dark and I haven't been in, So I can't tell you exactly how much has been damaged."
Mr Brockbank said he hoped smoke and water damage was not too significant in other areas.
The school was knocked around badly by Cyclone Ita last year, he said, costing roughly $150,000 in repairs.
"That really caused us a lot of damage...some areas we haven't got completely repaired yet."
Mr Brockbank, in his third year as principal, said tonight's incident was a "major setback" for the school.
"We have got some older buildings and the tragedy of this is, it's the one area of school that had been revamped in relatively recent times."
The school would be closed tomorrow, he said.
Mr Brockbank said there was initial relief in that no one was at the school at the time.
But it was still "very, very disappointing", he said.
"We didn't need this again."
A Fire Service spokesman said a fire investigator would be at the school tomorrow and that until then there was no indication as to what may have caused the blaze.