"Police are working to understand the circumstances," the spokeswoman said.
A Fire and Emergency shift manager said crews were called about 4.45pm.
Two crews from Middlemarch and three crews from Ranfurly attended the fire.
"We were alerted to a report of trees on fire. We have got our crews from Middlemarch and Ranfurly in attendance at the moment. The fire is contained."
He said the blaze was burning through a small amount of firewood and 20 metres of trees.
"Our fire investigator is on scene at the moment."