if your 156 is still under warranty, don't go anywhere but an authorised Alfa workshop.  The cost isnt that high and where to go depends probably on what suburb suits you best.  Lance Dixon is in Doncaster, Robert Lane in Brighton and Zagame in West Melbourne now.
The Italian Job in Oakleigh can reset the computer too i believe.  Some others like Monza in Bayswater may be able to do that too.

If it is out of warranty, still seriously  consider taking to one of the authorised mechanics.  They know the car better than anyone else.

Any of the Alfa dealers will tell you the service cost up front if you call them.  Materials extra of course.