Looked into buying a wrx from over east when I was looking, rang WA licencing and sydney and melbourne centres to ask about this question. Unless you have a registered local address in that state, you cannot pay the stamp duty there. The WA licencing chick said they really don't care, but the eastern staters were really strong on it.....

Shame cause they pay WAYY less than us.. (but get shafted with RWC's, and having to put their cars over the pits regularly....