These are the regulations as found under the heading of "Vehicle Condition" on the VicRoads website.
It is important to have adequate rear vision when the caravan is attached. Depending on the width of the caravan, you may need to fit extension mirrors to the towing vehicle. Extension mirrors are available from most caravan retailers and auto shops.
Visibility is also improved by ensuring that the windscreen is clean both inside and outside and that objects or stickers that obstruct your view are removed.