Welshwife there is nothing to prevent trading with Australia now, just as there is nothing to prevent us buying NZ lamb and dairy products (Anchor brand) and wine.
In fact we do, of course, drink megalitres of Australian wine every year. And they, presumably, drink Scottish whiskey if it is their tipple of preference. When visiting there I don't see much evidence of them buying much from us other than niche food products (Marmite for expats...)
The prices would be slightly different if we had a different arrangement with them over trade.
One of the interesting things I notice about Australia is that if you go to the town centre or the mall, hardly any of the shops are familiar brands from UK or USA. Same in NZ. Really not worth major chains starting up there, I guess, as it is so far away, and where clothes are concerned, their seasons are opposite to ours.