Generally, the delineation between lot property and common property is 3mbodied in the strata plan, 2hich form part of the records.
Upon purchase each owner has a copy attached to the contract of sale.
If one is not at hand then jt can be obtained from LPS.
Sometimes the plan is badly drawn, and there is ambiguity of the dìvision.
This is an issue that NCAT can’t sort out as its the jurisdiction of the land and environment court.
If it is in fact that the plan is ambiguous, it may pay both parties to consult a lawyer to provide some guidance. This is a fairly specialised area so your local solicitor may not be the best.