It is a salvage mission in international waters, China should take over the search
Australia only had exclusive jurisdiction when it was a Search and Rescue mission.
For a long while now it has been a salvage mission in international waters, anyone or any nation can do that.
China or Malaysia should take over the search, because they have human reasons for wanting the salvage. (Or the USA, the aircraft manufacturer being based in the USA gives the USA technical reasons to want the salvage.)
The rescue mission was completed long ago. Australia has no greater responsibilities in the salvage mission than India or South Africa or Canada does.
For the sake of the families, I hope China does the right thing and starts planning its salvage mission immediately.