Tax offices don't work on a people reviewing number basis any more, they use data analytics to identify discrepancies, and automation to send you a claim / penalty. So anyone who tells the UK HMRC they are in Australia, HMRC will share their data with the ATO in a bulk file, who can run a single exercise to flag the missing income. They can pick up literally everyone. And they will. Once they have a discrepancy it IS worth their time following up.