Basic insurance is included in the supplier rates, however, in the event of damage to the rental car, each supplier has a damage excess fee. The damage excess fee varies for each supplier, and is around $3000 (higher for specialty/luxury fleet). This is the amount the supplier will charge (up to), if there is damage to their vehicle. Costs are covered after that excess damage fee amount.
When you have decided on the supplier details are displayed at the top of their Terms and Conditions of rental "Included in this quote". Their terms will display the excess fee specific to that supplier.
Excess reduction from the supplier can be accepted at the counter - the most convenient way for worry free travel. There are varying levels of protection that will be offered at the counter - excess reduction, and also the full protection packaging. Refer to the terms and conditions of the suppliers (from around $30 per day). In the unfortunate event that there is damage to the vehicle, the supplier will charge up to the damage excess fee on the contract regardless of your own personal cover.
The excess reduction is not compulsory in Australia. It is an optional extra offered to you when the vehicle is collected. If there is damage to the vehicle, the car rental companies will charge the excess on the Rental Agreement, if any damage is identified at the return of the vehicle or soon after. Your personal insurance circumstance is not taken into account, until you make claim with them, and it is processed.