Or, on the West side:
Esperance - for the Sanko Harvest, the third largest diveable wreck in the world.
Bremner Bay - for Leafy Seadragons.
Albany - for the Perth, a former navy destroyer scuttled for divers.
Dunsborough - for the Swan, a former navy Destroyer Escort scuttled for divers.
Bussleton - 2nd longest jetty in the southern hemisphere, loads of life.
Bunbury - for the Lena, a former fishing boat, confiscated for illegal fishing and sunk for divers.
Rottnest Island - Some of the southernmost hard coral reefs that you can find, plus shipwrecks, turtles, occasional whales etc.
Fremantle - for Weedy Seadragons, Seahorses, Dolphins etc.
Jurian Bay - for Sealions.
Abrolohos Islands - Deserted islands with loads of life.
Coral bay - for Manta Rays, coral reefs and sharks
Exmouth - for Whale sharks, Ningaloo reef, (better than the GBR, IMHO) and the Navy Pier, (one of the top shore dives in the world)
Rowley Shoals - offshore liveaboard diving.
Broome - Tropical waters plus the chance of being eaten by a Saltwater Crocodile!

All this plus we are a long way from Sydney