The signal is picked up by Emergency Services and Police or SES officers will be able to locate you. A comprehensive first aid kit. Phones are few and far between, and the service truck doesn't usually go that far anyway. In general, the limit is 15-30km outside of towns with network coverage. However, satellite phones can be used from anywhere, and although expensive to buy and use, you may like to consider one.
Flash flooding can close a road within minutes.
Even some of the major highways can be affected by this. Also, many roads are routinely closed during the summer 'Wet Season' in the northern parts of Australia. When you arrive, call back to let them know you have arrived safely. If you do not call them on the expected time of arrival they can notify emergency services. If you don't have any family or friends, leave your details with police.