A fire which destroyed an outbuilding at an Auckland primary school overnight is being treated as suspicious.

Firefighters were called about 4.30am to Wesley Primary School in Mt Roskill, southwest of the city centre.

Upon arrival, they discovered the small outbuilding well ablaze.

"It was brought under control fairly quickly but there's not much left of it," Fire Service northern communications centre shift manager Jaron Phillips said.

- NZPA