Have you checked the Body Corp and relevent by-laws allow for separate leasing of the unit and car spaces?
Have read that this is sometimes specifically denied in some strata units such as in the CBD etc.
Our rules, from memory, state that spaces can only be rented to people who live in the building. It's a rule that cannot be enforced, but, as implied above, renting a space to an outsider lessens the building security in that you would have to give your car space tenant a security tab which - in our building anyway - gives them free access to the residential common parts, post room and swimming pool.