Martyn asked….. Is it illegal for a tenant (or their agents) to advertise over occupancy and/or would a by-law to that effect hold any validity?
To the extent that a Residential Tenancy Agreement (NSW) must show all persons residing at the premises as tenants, and requires those tenants to not permit any additional persons to reside there; YES it would be illegal for a tenant to advertise for additional occupants unless the Owner / Landlord agreed.
In terms of possible over-occupancy, for reasons including fire safety (exiting), vehicular parking, the possible overuse of common facilities, and anything else that could be justified an Owners Corporation (O/C) could validly determine and Register a Special By-Law limiting the maximum numbers of adult persons permitted to occupy any Lot, typically to a number equal to twice the numbers of bedrooms.
Compliance with a Tenancy Agreement is regulated by the NSW Office of Fair Trading and the NSW Civil and Administrative Tribunal (NCAT) but regrettably an O/C is not a “person” entitled to lodge a Complaint about breaches of those Agreements, however compliance with any Registered By-Law is enforceable by an O/C through the NCAT (incorporating the former Consumer, Trader, and Tenancy Tribunal).