Looked up on Telstra website and this was the information they provided.
“Your IP has changed from Private to Public” scam
In this telephone scam, the caller tries to convince you that your home network, and your IP address, has switched from a “private” to “public” mode, and that you are at risk. This is a trick to try to convince you to let the caller have remote access to your computer and home network. As always, Telstra advises you to immediately hang up on any such caller.