I love Park Hyatt Sydney. It's truly an aspirational property. The location is great for access to the ferries and sightseeing boats. Still, I had an excellent stay at the Sheraton on the Park. The train from the airport has a stop immediately across the street. It was an easy location for transportation as was the Westin where I have also stayed.
PH Melbourne is ok, but not on the same level as the PH Sydney.