Long-term monitoring has revealed over 100 species of coral-reef fish expatriating into SE Australian waters to rocky reef habitats (“vagrants”). Most species are transient, that is they do not overwinter. However, a recent expansion of branching coral in temperate SE Australia has opened the door for a number of new arrivals and allowed more species to persist. Pocillopoa aliciae, a subtropical hard coral that has rapidly expanded over temperate reef “urchin barrens” in the last few years, has expended in several sites along the Sydney coast and in Sydney Harbour. We show that an additional 8 species of vagrant fish have now been recorded in Sydney, only on these coral beds, which also support a much greater overall density of vagrant species (and some temperate species) than surrounding rocky reefs. Even small patches of P. aliciae support an order of magnitude more reef fishes (tropical and temperate) than surrounding rocky reef. However, while the coral has generally expanded at monitored sites, in one case (Dobroyd Point) we have seen a decline, possibly due to erosion by urchins. Tropicalisation of temperate waters can therefore be enhanced by expanding habitats (in this case, subtopia corals) which can facilitate establishment of coral-reef species otherwise unsuited to local temperate habitats. We estimate, based on these data, how reef fish assemblages in the Sydney region may change under future climate change projections.