Fire and Emergency NZ area manager Steven Greenyer confirmed they are investigating how the appliance was destroyed on Monday.
Crews were called about 4.30pm after a harvester caught fire on Stanley Rd.
"None of our people were hurt and the fire was extinguished by about 6pm."
The fire engine, which Stuff has reported is understood to be worth about $500,000, was based at Ashburton.
"We are working with our people who were at the fire to understand what occurred."