This is definitely enough time. You won't need to claim your bags if you can get the airline to interline your luggage, which shouldn't be a problem (isn't Etihad the only airline to fly from AUH to SYD or MXP anyway?).

If you do manage to get your bags sent all the way, then even if you don't get the onward boarding pass in SYD, there is a transfer desk in the transit area, so you can get your MXP boarding pass and will not have to go through immigration. Security is at the gates in AUH, so you will exit the plane directly in the transit area. It's a small airport, so you will have no problem.

In fact, even if you have to go through immigration, retrieve your luggage, check in again, and go back through immigration, this should be enough time. It would be cutting it a bit close, but I wouldn't worry given the size of the airport. The fact that most passengers through AUH are transiting means that the waits at immigration and baggage claim should be modest. They always have been in my countless times at that airport.

Edited to add: One caveat; everything above refers to Terminal 1. It looks like security in T3 is centralized and not at each gate area. Still, you should not have a problem.