CTP (Comprehensive Third Party) is a must. It's also called the "Green Slip" and a requirement for registering your car with the RTA.
No CTP Greenslip, no rego.

There are two "optional" insurance above the CTP.
Third Party Property Insurance - this covers any damage to other property your vehicle would hit (cars, boats, houses etc). You would be very much advised to take out this insurance if you don't want to pay for the repairs for hitting someones Lambo or Aston Martin. It won't cover your vehicle - which would be old, second hand and the cost of insurance would be worth more than the car itself (eg. your car is under $5K)

Comprehensive Insurance - covers other property and your car.
This covers anything you hit and your car too (less excess) - it's more expensive, but then again, writing off your car is expensive to replace. Usually take it out if your car is worth more than $10K.

Other additional bits of insurance to add
Windscreen protection - this additional cost to insurance is mainly for people who do a lot of highway driving. It's an additional $50 a year, but if your windscreen is cracked/smashed from rocks, then it's replaced (up to a number of times - like 2-3 times) an no excess paid. Had it on my previous car for 3 years ($50/year) and replaced the windscreen by O'briens once (cost of $350 covered by insurance - and no excess paid)

Car rental allowance - an additional cost if you cannot go without a car and have no other cars to beg/borrow/steal (okay, not the steal bit!). It would allow you to hire a car at a "special rate" for a period of time. Never had it or used it.