That's no good indeed!
I hope the following link helps. Lost-stolen-or-found-your-mobile-device
Yep, nothing less fun that paying for something you don't have.
@caaf gave some good practical tips.
When you contact Optus they should be able to tell you how much finalising the contract will cost. Often there's no or only a little cancellation fee on the plan itself. You'll just need to pay the remaining handset fees part. optus usually subsidise the handset fee while you are on contract
Usually a better option is to just buy/borrow another phone and Optus will give you another SIM and you just continue off paying as you did before. The contract ends and you are done - you just don't have the phone that was stolen. You could get a pretty good cheap one for under $300. Second hand on eBay?
I'm not a fan of Extended Warranties etc but with phones its probably worth paying the insurance. Optus has its offering but I'd also look to see if you can't insure your phone under your home contents insurance (if that's an option)