Ok it looks like in NSW you don't get a registration tag like you do here. They send you a Certificate of Registration and the law states
http://www.cityofsydney.nsw.gov.au/R...gOwnersNSW.pdfPlease note: your dog is required by law, when in a public
place, to have a collar around its neck with a
tag attached that shows the dog’s name and
the address or phone number of the dog’s
So if you see this dog again see if it has a tag with its details on it (including address etc) and report that to the Ranger and if it doesn't call the Ranger and report the description of the dog and the fact it is not wearing any ID.