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The man, a neighbour of the property owner, was sweeping leaves off the garage's flat concrete roof when it gave way and he fell about 2m to the ground.
As the garage, one of three in a block, was surrounded by trees, the roofs were often cleared of leaves during autumn and spring.
The man, who did not wish to be named, said he suffered a few cuts and scrapes but was otherwise unhurt.
An appliance from the Oamaru Volunteer Fire Brigade attended the scene shortly before 5pm to manage traffic while one lane of Chelmer St was closed due to concerns the garage might collapse.
Chief fire officer Steve Couper, of the Oamaru Volunteer Fire Brigade, said consideration was given to demolishing the facade of the garage as ''not much was holding it together''.
However, it was instead reinforced with timber as a temporary measure.
Property owner Donna Demente said she was not home at the time of the collapse but rushed to the scene when called.
''I was glad I was not on it when it happened,'' she said.
''I go on the roof and sweep leaves off it and stuff, so I'm glad it didn't happen to me. Luckily our bikes weren't in there and it was just junk.''
Ms Demente said the structure was insured and the collapse may have been a ''blessing in disguise'', as she had planned to replace or upgrade it.