Police joined one crew from Fire and Emergency New Zealand (Fenz) at the fire in felled trees, intended for firewood, at the edge of a sports field at John McGlashan College early yesterday morning.
Fenz had been alerted to the fire about 5.30am after a caller reported seeing smoke in the area.
A spokeswoman for Fenz said that upon arrival, firefighters found a bonfire which was "burning well".
Roslyn Station fire officer Martin Jansen, who attended the scene, said the trees had been felled six weeks ago, to ensure the safety of the school’s pupils.
The resulting logs were waiting to be cut and sold as firewood by the school, he said.
Mr Jansen said the fire was being treated as potentially suspicious.
Police attended the scene, and had spoken to the school groundskeeper.
By 9am the fire had been extinguished, but Roslyn fire crews continued using ground monitors to ensure it did not reignite.
Mr Jansen said the crew had stayed at the site to ensure the wood was sufficiently wet to pose no further danger.
"Ground fires can burn for weeks if left unchecked — and at this time of year especially, it's vital fires like this are completely extinguished," Mr Jansen said.
"As always, if you see anything suspicious over the holidays, call 111."
