The worker was crushed by a corroded gantry bridge.
The incident happened at Karara's mine in Western Australia's Mid West in 2015.
Seven workers and members of the public were in the exclusion zone at the time but escaped injury.
After adjusting the mirror the worker fell 2.7m onto his head.
Regulator says plant and equipment must be fit for purpose.
A supervisor at the Keysbrook mine faces the music for an injury that happened on his watch.