Two crews from the Oamaru Volunteer Fire Brigade were called to the scene in Regina Lane about 3.30pm. They were followed by another crew from the Waitaki Rural Fire Force a short time later.
Fire and Emergency NZ shift supervisor Brent Dunn said a shed was "well ablaze'' when firefighters arrived.
Fire appliances gained access to the shed, in a large, vacant, overgrown section, through the yard of Peter Murdoch Transport Ltd.
About a dozen firefighters extinguished the blaze, some in full breathing apparatus. It took about 30 minutes to bring it under control.
A small section of vegetation next to the shed was also burnt.
The section is beside railway lines and a track used frequently by walkers and cyclists.
Sergeant Craig Sew Hoy, of Oamaru, said it was not clear how the fire started, but confirmed it was being treated as suspicious.
It was not believed anyone was in the building at the time of the blaze, he said.
An investigation into the cause of the fire had been launched.
- Oamaru police (03)433-1400; Crimestoppers 0800555111.
