1. In my reading of car rental contracts in Oz, typically you need a 4x4 or AWD to be allowed to drive off sealed roads.
2. Many companies have further restrictions- like Bloomfield track, or other noted roads or regions. The local company will usually list these on your agreement/contract
3. While rental companies may not permit the use of the car off road, the restrictions around 'night driving' CAN BE more nuanced. SOME allow you to drive at night, BUT you will be 100% responsible if there is an accident. This can be an important distinction- if you are using a car in a way that is not permitted by the contract, you might find you have no cover- Nothing with the company AND nothing with your own policy or with your credit card. However if you are simply driving at night, which is permitted, but they dont cover an accident, your travel policy or CC policy may still cover. (This was the case when I drove a hertz 4x4 car on KI in 2016- it may have changed since then, although what is on a web page isnt binding, so read the fine print)