Hi Dan,
I live in the hinterland of the Gold Coast so I am guessing that I have similar seeing conditions as Caboolture with the exception that my view to the south is superior to the view north and your's would be the other way.
I believe that the best test is judging what you can see with the naked eye. For example, on a very very good moonless winter's night, I can just make out the Lagoon Nebula, albeit through averted vision and after a good hour of dark adjustment. I can clearly see Omega Centauri on a moonless night. That said, I have never seen m31 with the naked eye because of the light pollution from Brisbane to the north.
You might be interested in the following link that explains the "Bortle Scale" for judging light pollution:
http://en.wikipedia.org/wiki/Bortle_scale
Duncan