FYI, the number being unavailable or disconnected when you attempt to call back is a sign that they were using a dialer with a spoofed Australian number for the caller ID. One of the biggest signs of a scam call there is, and I would wager the caller was not even in Australia.

This is the second Sydney number to call me in the last hour or so, on a Saturday, and I am in Perth. I do not answer numbers I don't recognise and wait to see if a message is left. No message was left in this case so I called back using the 1831 prefix (which blocks your number from showing). I got a recorded message saying, "The number you have called is invalid or incomplete, please check the number again". I tried again with no 1831 and got the same message. Pretty sus - I would say scammers who have somehow been able to disconnect their number from being traced or logged as in use by someone.