Male Australian sea lions are not the same size as female elephant seals when it comes to length. Sea lions (being an otarid) have a different use of water space to phocids. Mac, Maude and Nella died a number of years ago. The Macquarie Island exhibit was purpose-built for them, and ultimately not suitable.They are not a species that are suited to Sydney conditions and I personally would not like to see them on display in Australia. Pet Porpoise Pool had a female called Sophie for a number of years.
Vagrant southern elephant seals do turn up from time to time on Australian shores. Taronga rescued, rehab'ed and released (complete with satellite traker) a young female, and later rescued an older female that sadly died.
Demon the bottlenose dolphin was at Taronga for a few months in 1991. As Jones said, he came from the African Lion Safari (the last surviving dolphin there) and was shipped to Sea World, where he lived for quite few more years. He was actually from QLD originally.
As for leopard seals, it can be argued whether this species is suitable for captivity, particularly when there is a failure to replicate their natural environmental conditions for (some unknown reason).