Lies, it is a scam.

Caller: 1800 305 499

This is the ATO's number. I got the call and it sounded like a scam, so I called the ATO 'scam line'. They had no record of the number, which was concerning, until they looked up my file and there was a record of someone from the ATO calling me. So they must use an outside number?? Pretty poor performance on the ATO's part that they have not recorded the number so we can verify it.
But when you call the number back, you get through to the ATO.

This is the TAX dept ATO 1800 305 499

Left message on voice mail to call them with my sisters TFN details regarding our account. Was told to call by close of business day on Wed. As I do have an account with my sister I called them. Sounded like the ATO, they even had waiting periods. Suddenly asked myself, "Why would the ATO be asking me for my sister's TFN?" Told them to get stuffed. This is a scam. They are getting too clever!

This is the ATO

Don't give out any information about yourself. This has scam written all over it.

Checked ATO on their 13 number.
Result - the 1800 no. above is ok.

Kay above might be someone working behind this scam.
Confirmed with ATO, this is not a number from them.

Its a legit number for the ATO, doesn't always work when calling back, can be cut off. I've spoken to the ATO to confirm it is one of their external lines.

It is genuine

I had same message left on my mobile

Followed up via ATO, on a different number than was left on the message.