some places in NZ and I'd guess Australia do have reduced entry fees for locals (city not country) for which they need to produce some form of proof such as a bill (i.e. proof of address). You do run into problems in the case of visitors who don't have proof on them - do you just take their word for it and thereby negate the system because it is so easily abused, or do you say they must have proof and thereby alienate your local visitors who feel aggrieved because you don't believe their word? And there's also the very real problem that you also lose out on tourist numbers because a lot of them feel ripped off because they are paying more than the locals and so don't visit at all. For places that have a flat entry fee for all (i.e. the majority of places), the yearly memberships are the equivalent of reduced local rates.