The Unit may only be sub-let by a Tenant with the prior written consent of the Owner, who may require an Agreement with the Sub-Tenant.
If the Owners will not do anything about the unauthorised sub-letting of their Units, then ask your Strata Manager to write to the Managing (Rental) Agents on behalf of the Owners Corporation (O/C), pointing out that the head tenant/s are in breach of their Lease Agreement.
A Special By-Law to prevent sub-letting would be difficult (if not impossible), BUT if overcrowding is also an issue, then your Owners Corporation could by Special Resolution at a General Meeting create and Register a Special By-Law to limit the numbers of adult people who can occupy any Unit. To be sustainable and enforceable that number should be based upon something known, such as the numbers and sizes of bedrooms.