AUCKLAND, Aug 6 - A toilet block at Okaihau College in the Far North caught fire on Thursday morning, and when the Fire Service arrived they found it was smoke-logged, Jaron Phillips told NZ Newswire.

About five students at the school who volunteer for the local fire brigade helped put it out, rolling hoses and doing other things that were required, the school's acting principal Karen Campbell told NZ Newswire.

All classrooms at the school which has a roll of about 500 students were evacuated and the students had an extended interval break, she said.

The fire is being treated as suspicious, but Campbell wouldn't comment on whether a student had been involved, or how the fire had started.

The toilet block was damaged in the fire and won't be used until it's repaired.

She said fires at the school weren't a common occurrence, although one had been lit around five years ago.

A fire investigator is examining the scene.