A garage is not a habitable space for the purposes of the Building Code of Australia. Different building standards regarding things such as waterproofing, light and ventilation apply to habitable and non-habitable spaces.
You would need to consult a professional regarding the changes you would need to make to the garage that would meet BCA requirements for a habitable space. This may or may not be possible, or the changes required may be considerable. You would then need to apply to the Council for consent to use it as habitable space.
The question is not one for the Owners Corporation, although if you put a DA in to convert the garage, you would need to seek their consent.