David is correct. Your licence is valid (due to the recipricol agreement with Oz) for 12 months from the date of your entry into the UK. If you plan on staying more than 12 months, you must pass a UK driving test before that 12 months is up. If you went back to Oz after, say, 11 months and came back a month later, the 12 months would start again from your second date of entry to the UK.