I am going through the process at the moment. Basically to convert an overseas ATPL there are 3 steps:
1 - Apply to CASA for a validation of overseas qualifications
2 - Pass CPL Airlaw and IR written tests and then take the CPL and IR Checkride (a flight school wanted me to demonstrate to a flight instructor with a fraction of my experience that I was able to fly VFR in Australia without getting lost and to shoot an NDB approach)
3 - Pass the ATPL Airlaw and bring it to CASA, pay a good amount of money and that's it.
I have done steps 1 and 2. I'll let you know about step 3 in a week or so.
Note that IR written in not as easy as in the US, but for the checkride all you need is an instrument departure and an NDB approach. That will be enough to have the rating, but if you want to fly the other approaches, than you have to be tested in any single of them!
Every year also you need to pass an instrument renewal test on every approach you are endorsed for... no matter if you have ATPL, IR seems to be a sort of separate thing on your license, and if you let it expire for longer than 12 months you have to redo the written as well!