Well, that didn't take as long as I thought.
The tally of Federated Search/Discovery Layer tools in use in all 39 Australian universities looks like this:
Not sure that the increased popularity of Primo in non-law teaching universities is causal or not.
Even with universities using the same platform there is not a cookie cutter approach. The use of the tools varies widely in terms of content and presentation. University of Western Sydney for example, keep their catalogue records out of Summon and use Aquabrowser to visualise the catalogue search results. The University of Ballarat presents both Metalib 'interfaces' as two separate services: Quicksearch across generic databases and federated searches across databases grouped by subject.