I'm from New Zealand, where "Master" is used to address young boys, often by a doctor/dentist etc. I guess that it's the same in Australia and the UK. Do not address an adult (in any situation) as Master Smith. Conversely (as suggested in the comments), addressing someone as "Master" without being followed by a name, such as "yes, Master!", sounds like you're addressing a slavemaster.
The formal and safe way is to address them according to their rank - Mister Smith for teaching assistants who are Master or PhD students, Doctor Smith for postdocs and staff who hold a lecturer position, and Professor Smith for associate or full professors. The abbreviation Mr. is fine for emails, as this is commonly understood as the abbreviation of Mister, not Master.
However, Australians are generally quite informal, so the younger staff in particular may prefer to be addressed on a first-name basis. Calling your TA Mr Smith might come across as a bit fusty, so they might tell you to "just call me John". If in doubt, just ask how the lecturer wishes to be addressed.