I don't about the Australia issue, but there are a couple vendors on Ebay
selling Lexus knockoffs.

At least in the states, the 17-inch wheels were available on the GS400/430
only.  Most guys who want the 17's for the GS300 find takeoffs from folks
who have upgraded their 400/430s with aftermarket wheels.  clublexus.com
always has a few people selling theirs.  Again, I don't now what issues you
have with getting stuff to Australia.

Personally, the Lexus wheels are Okay, but nothing to write home about.  If
you're looking for more bang-for-the-buck, I'd go aftermarket.  Or leave the
16's on the ground and go slightly wider and slightly lower-profile in a
front-line tire like a Michelin Pilot Sport.  You'll get 95% of the handling
of the 17's and keep a better ride.

- Mark