Late to the party, but whatever.
There are sites in Australia that are on very well-known but enclosed private property. The only way you can physically access those sites, is by knowing the local protocols.
For example, at one site, you have to park outside a farmer’s house - keeping off his carefully manicured grass! - knock on the back door (not front door), show him your card, and get the key, before you can access the paddock containing the site.
There’s another site (Tank Cave) that is actually owned by the Cave Diving Association of Australia (CDAA). The entrance to that site is protected by a locked grate, which is only unlocked by an authorised CDAA officer, for pre-booked CDAA members, at the start of each diving day.
In such cases, if you don’t have the right certification, you can’t even access the sites - let alone dive them.
There are other sites (in the same areas as those above)
that need you to present your qualifications, and pay a fee, to a government authority. There’s nothing to physically stop you diving most of those sites, but there’s a ranger who sometimes drives around and checks people out. Not sure what would happen if he found unauthorised people diving them!
In summary, all of this is clearly country and site dependent. There might be a publicly accessible site, that any unqualified random person could dive - 50 meters away from a private site, that only properly qualified people could even get physical access to.