If you visit these capital cities in Australia, these are must see places, in order of importance.
Melbourne has St Kilda Beach. Catch a tram to get there.
Sydney has Manly Beach and Bondi Beach. Catch a ferry to get to the first, catch a train and then a bus to the latter, if you can squeeze onto the bus. Maybe better to get a bus all the way.
Adelaide has Glenelg Beach. Catch a tram to get there.
Perth has Fremantle. Catch a train to get there. Also Cottesloe Beach, catch a bus.
Darwin has Mindil Beach. Catch a bus to get there. Hundreds do, to see the spectacular sunset. Don't be a dick like me and ask the bus driver to tell me when we arrived at Mindil Beach. The very full bus emptied at the bus stop.
Brisbane is a bit inland on the Brisbane River. I don't know where people would head to a beach there, and the waters at a beach there could have nasty stinging things. Educate me! Is Fortitude Valley the tourist hotspot in Brisbane?
The same with Hobart. It too is on the Derwent River. I suppose you would say Salamanca Place is the tourist hotspot, but there is no beach.
These are all my photos taken from my blog and do not represent the best of what you will see.
Later edit: At Brisbane's Southbank is a constructed beach and from the photos I just looked at, it seems great.