Check your insurance.
You are insured on private property within reason and covered by your personal vehicle insurance and the property owners property insurance.
If they state it is not a gazetted road that are absolving themselves of your insurance coverage.
That is the point of the statement
otherwise they would need to get full public liability insurance.
Of the policies that have a clear PDS, or of which we could get clarification, NRMA Insurance, CGU Insurance, Club 4×4 (underwritten by The Hollard Insurance Company), Affinity Insurance Brokers and DGA Insurance (also a broker) will provide coverage for gazetted and ungazetted roads. So if you’re off-roading on private property, for example, and have a crash, you’re covered (provided you have permission to be there).