Les,
My understanding was that a car that is efi'd an isolator switch (such as a battery isolator) is illegal - hidden or otherwise.
All new cars must be fitted with an immobiliser of some sort.
Watch out for insurance - I know Shannon's require proof that an immobiliser is fitted (and it needs to be a recognised type) prior to reducing theft excess.
All of the above may be unique to QLD that being we're all different countries in one
if you know what I mean!