An automated message along the lines of "This is the Department of Australian Tax Office. Your tax file number has been suspended. If want to discuss further then please press '1'." I didn't bother pressing '1' and am assuming this is a scam.

....and I forgot to mention that I reported this with the ATO, i.e. https://www.ato.gov.au/general/online-services/identity-security/verify-or-report-a-scam/