Neil, do you know that most of us are huddled along the green strip on the right hand side, by the white sandy beaches? Once you get away from this bit there is very little traffic. There is a nice range of mountains - the Great Dividing Range - not far from the eastern edge. If you like twisty mountain roads this bit will please you immensely. Beyond this, heading west, it gets a bit flat and the roads get very straight.
As indicated above, March / April (our fall) is a good time - more predictably dry than spring, and with comfortable temperatures. I hope you will come to the region where I live (ME109 and Doug and a few others live nearby - google 'Albury'). I look forward to meeting you, and there are some spectacular mountain roads nearby. I'm sure we'll be able to arrange a gathering to coincide, perhaps involving a ride. There are also other mobs of boxerworks blokes further north, most of whom sensibly live in or near the hills. Check Rob's map - where the flags are is typically very good riding.
Three weeks seems very brief. Why not make it eight weeks (or more) so you can do some decent exploring? There are jazz and blues festivals at various times and places that you may wish to catch (google will help). These are typically held during that nice autumn period because of the predictable weather.
Surfing happens all around the coast, except at a place called Surfer's Paradise, which should be avoided at all costs. Sydney has some great beaches, and there are lots more right up the coast from there.
and Brunhilde - 1974 R90/6
Graduate, Wallace and Gromit School of Engineering and Design (Pending)