Generally on major highways, fuel stops are rarely more than 200 kilometres apart so it should not be necessary to carry spare fuel in your campervan. If you are hiring a campervan it is a breach of your terms and conditions to carry spare fuel, plus in particular if it’s petrol it is extremely dangerous.
However, where you do see ‘no fuel’ signs, it means exactly that, so a great time to fill up. The signs indicate how many km’s to the next station. Also be mindful if you are travelling in remote areas of Australia the fuel stops will not be 24 hour like you may be used to in larger cities.
You should always carry water, a first aid kit and ensure that your spare tyre is at the correct pressure.