Crews from the Wanaka and Lake Hawea volunteer fire brigades were called to the fire in Camphill Rd about 6pm on Tuesday.
Lake Hawea Chief Fire Officer Brent Arthur said the two-story garage was well ablaze by the time crews arrived.
There were no suspicious circumstances surrounding the fire, Mr Arthur said.
The fire proved ''tricky'' to extinguish because of concerns about the structural integrity of the garage and the large amount of flammable objects inside.
''The likes of garages are cram-packed full of all sorts of things and this one had your fuels and the rubbers and the plastics from the vehicles.''
Mr Arthur said he was reluctant to send anybody into the building once they knew nobody was inside, so the fire had to be ''attacked'' externally.
The garage was not connected to the house but the upstairs area of the garage was used as a sleepout.
No-one was injured.
A fire investigator visited the site yesterday morning but the cause of the fire was still to be determined, Mr Arthur said.
