Just searching on resources on the internet (assuming you are based in Sydney - as you did not mention this),
I found using Krisflyer miles, you can redeem Star alliance redemptions to get to ZAD for 50k in economy 1 way. You may have to pay some tax/surcharge so it might be worth while to do it in business class as you will be paying for the same amount of tax/surcharge (usually) and you do have the required miles/points. In business class, it costs 95k. Some example of routes are SYD-BKK-MUC-ZAD or SYD-PEK-WAW-ZAD.
If you are targeting SYD-ZAG instead with points and making your way to ZAD in a cheap domestic ticket (if there is such a thing - I am not very familiar with Croatia), you could use Qantas points or Asia Miles to fly SYD-DOH-ZAG for 110k Asia Miles or 139k Qantas points in business. 70k AM and 75k Qantas points in economy.
I think first thing you have to check is availability. If the dates are already available for booking (check on Qatar air website whether the dates are already open for booking), I would suggest checking on Qantas website (click classic reward) and try finding from your city to ZAG. You should be able to see Qatar air seats if available and book online through the website.
To check star alliance availability, United.com is a good and user friendly interface. Once you find the seats, you will have to ring up Krisflyer and ask them whether they can see the seats. They will give you the estimated tax/surcharge that you need to pay too. To get Krisflyer, you will have to transfer your Velocity points (instant) or Amex points (a few days).
Hope that helps.