In the 16 inch you can get Yokohama Advan Neova AD08R's in the correct sizing :)

According to the rules, wheels must be oem sizes for unmodified street class, so i might as well get 17's rather than 16's. I imagine it wont be long before 16's are hard to get too.

Yeah, 16 inch sizes are limited too, mainly for the rears if you want to retain the rolling diam.
 
BTW, just checked with Geelong Jaxfit for 225/50R15's
Pirelli P Zero available $330 ea (ouch) 
 
But I have no idea of how these would work on the rear of an SW20.
 
 
 

Thanks everyone for your ideas and help, but it looks like i'm saving for some 17's....... 
 

Dunlop was tyres for 15's but 17's do sound like the right idea
 

Call up your tyre dealers and ask if the have any Toyo T1r's. They would do both front and rear in the correct sizes if they have them in stock.

I'm looking for something better than T1R's for the track days... T1R's are great in the wet, but it hardly ever rains here lol... R1R's would have been ok, but are not available in my sizes...
Thanks anyway...

A lot of people here will call it blasphemy for even suggesting it, but I can actually recommend going to eBay tyre dealers. There are a couple based in Sydney that have proven to be pretty good for me. I've used them twice for tyres that I've taken to the track and thrashed and they performed exceptionally well.
I did a quick search for your sizes and came up with a 205/50R15 for the RSR's, which I can personally vouch for, and the 225/50R15 offered up a Toyo R888, which are pretty good, or so I hear.
Or, as several people have said, move up to 16's or 17's, where tyres are easier to find, but also tend to cost more.
I can also guarantee that, even though you're up in Ceduna, it'll be cheaper to freight a set of tyres to your door from Sydney than to buy from your local dealer, as they mark up prices a LOT. Got quoted ~$300 per tyre for RSR's at a dealer, bought them for $155 each on eBay, and delivered for $50.
 
I'd suggest going to a dealer only to fit your tyres, and even then, take the wheels and tyres separate to the car. I've seen too many ruined studs and new unexplained dents after taking the whole car to trust them myself, and I work in the industry.
 
If you plan on keeping your original wheels, or even upgrading to a larger wheel, let me know when you're after tyres. I could probably hook you up and save you a few hundred :)

Or you could get Federal from an authorised dealer here on the forums who sell them at almost wholesale price for the benefit of club and forum members, and get them direct from the Australian importer ;)