well, Perth is a lot further from NZ and Fiji than Sydney is. There's quite a bit to see in the bottom left hand corner of the country - drive down to Albany or up the coast a way, visit Margaret river for the wines, swim with dolphins - but probably not 3 weeks' worth. I guess if I had to squeeze as much as possible into 3 weeks, it'd be Perth, Sydney, maybe Melbourne, Ayers Rock or the Barrier reef as side trips according to taste, Auckland, Rotorua thermal area, maybe Kaikoura for the whales and Queenstown for the bungee jumping and other adrenalin stuff, then a few days on a Fijian island to relax before going home... that suggests going east all the way on a round-the-world ticket but you could reverse the itinerary. This is realistic, but rushed.