Greetings to you in Down Under Land!
I'm about to go to Australia on business myself. Maybe we can link up sometime and chat about guns, ammo, Aussie culture, etc.
You can RELAX and be assured that extensive firing of .38 special ammo in .357 Magnum chambers won't hurt a thing!
I used to shoot PPC revolver, and consumed well over 100,00 rounds of .38 Spl ammo in my competition guns, all chambered for .357 Magnum. Ditto for 30 years of firing hot jacketed .38 specials in my Marlin M1894, Rossi M92, and many other S&W revolvers.
You do have to clean your chambers regularly though. I use a "Tornado" stainless spiral brush to get all the lead and powder fouling out. If you neglect this part of routine gun cleaning, you can get hard extraction and accuracy will fall off when you shoot .357 magnum ammo.
HTH
John