You've got to look at the odds here
What is the risk? What are the chances of having an accident, what is your car worth, are you shit behind the wheel?
If you are going to pay $2,500/pa with a $1500 excess on a car worth $12,000, Then you would have to have an accident in the first year with more than $4000 damage before it's worth it. In your second year if you have an accident your accident would have to do $6500 damage to break even and so on. In this scenario I would put my money in the bank and get 3rd party property insurance.
If your car is worth around 30K and the comprehensive insurance costs around $500/pa, get full comp
If you don't choose to have full comp, you must get 3rd party property insurance, unless you are in a position to replace a $100,000+ car you might crash into.
Insure what you can't afford to loose