If you are not insured and you receive a letter of demand this means that the other driver or their insurer is holding you responsible for the accident and is wanting compensation.
You need to think about whether you are fully or partially responsible for the accident. This is called liability. Liability can be shared between both drivers if they are both at fault. You should get legal advice about your liability.
You can ask for evidence of the amount of damage being claimed against you eg quotes or valuations.
When you have the information you should negotiate with the insurer or the other driver. You should get legal advice before you start negotiating.
If you also have damage to your own car from the accident and the other driver is partially to blame, you may have a counter claim, ie you claim against the other driver for the damage to your car when they claim against you. You should get legal advice about your liability if you think this applies to you.
If you can't reach an agreement you may go to court. If it is valued at less than $25,000 it may go to the Queensland Civil and Administrative Tribunal (QCAT). You should get legal advice about going to court.