The vast majority of visitors arrive and depart Sydney. This is not always the best idea, particularly if your trip won’t bring you back to Sydney. Finishing a trip in Perth only to fly back to Sydney to fly to Singapore means a 4 hour flight to Sydney to take a 8hr flight to Singapore rather than just taking a 4hr flight to Singapore direct from Perth.
Some distances to get you started:
Sydney to Perth 4110km or 2550miles (4 hours flying time), Sydney to Cairns is 3143km or 1950 miles (3 hours flying time)
Sydney to Melbourne is 9 hours if you drive up the Hume Highway, and see, well highway, or you could take a couple of weeks and actually see the gorgeous coastal scenery along the NSW and Victorian coasts on the Princess Highway.
Which is to say planning a trip across Australia and ending up in Perth doesn’t necessarily mean that you should return to Sydney for your international flight. Now that mainstream airlines have started to price tickets as two one-way tickets its no cheaper in general to buy a return ticket.