Common Property is defined in the Strata Schemes (Freehold Development) Act 1973 as ‘so much of the parcel as from time to time that is not comprised in any lot.’
Generally speaking, in most strata schemes, the lot owner owns the airspace and everything in it within the boundary of the unit. They don’t own the main structure of the building.
The owners corporation is responsible for the repair and maintenance of common property.
The following PDF provides a list of items found within a building that could generally be considered common property. However, depending on the history of the building, renovations and by-laws, determining what is and isn’t common property is a complex matter. In most cases you will need to refer to the strata plan and by-laws to determine who is responsible for repairs and maintenance.