When the premium is not mandated by the government, it is usually derived from the calculations of an actuary, based on statistical data. The premium can vary depending on many factors that are believed to affect the expected cost of future claims. Those factors can include the car characteristics, the coverage selected (deductible, limit, covered perils), the profile of the driver (age, gender, driving history) and the usage of the car (commute to work or not, predicted annual distance driven).
In South Australia, since July 2016, CTP is no longer provided by the Motor Accident Commission.The government has now licensed four private insurers - AAMI, Allianz, QBE and SGIC, to offer CTP insurance SA. The scheme allocates one provider for 3 years as part of vehicle registration. After July 2019, vehicle owners can choose a different CTP insurer and new insurers may also enter the market. 
Any car insurance comparison tool you look at should have your state’s minimum car insurance requirements pre-loaded into its options. States requiring PIP or medpay are generally referred to as “no-fault” states, meaning that when injuries occur, each driver in a crash makes a claim with their own insurance company to pay for them. Beyond the PIP or medpay limit, the at-fault driver’s liability insurance kicks in to cover the rest.
Police forces have the power to seize vehicles that do not have the necessary insurance in place, until the owner of the vehicle pays the fine and signs a new insurance policy. Driving without the necessary insurance for that vehicle is an offence that will be prosecuted by the police and will receive penalty. Same provision is applied when the vehicle is standing on a public road.
Auto insurance is financial protection, and not just for the investment you made when you bought your car. After a really serious accident, bills for damage and injuries can easily reach into hundreds of thousands of dollars. If you happen to cause such a wreck, the victims could sue you. In the worst case scenario, assets such as your savings and home could be seized.