Fire crews from the Redwood and Christchurch City stations were sent to the property on Innes Rd about 6.15am.
A Fire and Emergency New Zealand spokeswoman said the 6m by 8m garage was well ablaze.
She said there was no problem extinguishing the fire but the garage has been extensively damaged.
A sleep-out was attached to the garage but no one was injured in the fire.
A fire investigator was at the scene on Tuesday and the incident has been passed onto police.
It comes after two suspicious house fires also broke out in Christchurch on Sunday.
Fire crews were called to a suspicious fire at an abandoned house on Buckleys Rd, Linwood, about 9.50pm.
Earlier that day firefighters were also called to a blaze about 5.55am at a house on Fenchurch St, Northcote.
The blazes are being investigated.
