I find that really hard to understand.
My experience at YYWG has been one of totally friendly and helpful airport management.
Fuel is at least 10 cents per litre cheaper than the Yarra Valley, the shop carries lots of gear, a car is available at very cheap rental and you can grab a free coffee or tea.
It is a council-owned facility so they can make their own by-laws. Irrespective of ownership, I would be very careful following some of the advice already posted here.
In my view, if those signs are as stated, and you enter the airport airside, you have an implied invitation to do so. You have a right to use the airport but no right to do (criminal) damage to its infrastructure.
If you enter from outside without permission and have to break in to do so, you enter as a trespasser and can be charged with that as well as criminal damage.
It seems to me that a sensible course of action would have been to speak to the airport manager or, in his absence, the fuel guy who is also the GA Flying School proprietor to sort the issue out. It's not too hard to guess that the sign and penalty are there to protect aircraft owners from unauthorised access to facilities AND the visitor 's aircraft.
Kaz