unauthorised person, in relation to a place or thing, means a person who:
(a) is not authorised by the owner or person in control of the place or thing to have access to the place or thing; and
(b) has no other lawful reason to have access to the place or thing.
The key I believe here is authorised by the owner'.'
Usually these days the owner is the local Government Council, and they draft the access rules.
The aerodrome I operate from has a set of Local Govt Laws and Bylaws which govern access and use of the aerodrome by persons. These laws have been changed on occasion, and drafted following consultation with local operators and our Aerodrome Chamber o Commerce.
Consequently we have a reasonably workable aerodrome, unlike the cluster c..k you have to put up with.
Maybe you can get the Local laws changed if you want to.