A Fire and Emergency NZ (Fenz) spokeswoman said at 8.30pm the fire was about 400ha in size and burning on the eastern face of the Rock and Pillar range.
The fire was too dangerous for crews to approach, so they would be monitoring it overnight before calling in helicopters and other resources in the morning.
A Middlemarch resident reported the seemed to be "doubling every minute".
Another nearby resident said she believed it would be hard to put out due to severe wind in the area.
The fire looked to be "huge" and spreading rapidly down the mountain.
The fire is burning through the DOC Rock and Pillar Scenic Reserve and down towards neighbouring farm property behind McKinnon Rd.
A Fire and Emergency New Zealand spokeswoman said a crew from Middlemarch and three crews from Wakari were heading to the scene.
No properties were in danger.
Earlier, a farmer with stock near the site of the blaze was forced to attempt to move the animals.
The latest blaze comes on a day where the South has been ravaged by fires.
