Emergency services were called to the Lake Tekapo Tavern and Reflections Restaurant on State Highway 8 in the Mackenzie Country just after 1pm today.
Fire and Emergency spokesman Andrew Norris said the blaze was now contained - and they were just damping down the site.
It's a 600sq m building - and more than half of it has been severely damaged.
A fire investigator is now at the scene.
Video of the fire shows the building consumed by savage flames as fire crew worked to bring it under control with hoses.
Norris said the fire was "well involved" when crews arrived.
He was unsure if the restaurant had been open at the time but it had been evacuated by the time they arrived. There were no injuries reported.
The fire was not threatening other buildings, Norris said.
There were four fire engines at the scene and heavy equipment was also called in to enable them to access the roof.
