While in general you should be extremely skeptical about anyone claiming to be from the ATO, I contacted them through one of their published numbers after receiving a call from this number, and it appears to be genuine (ie, not a scam).

Claim to come from ATO and wanting to give $900 for the 2009 tax rebate. Trying to extract my personal information. Do not believe anybody claiming to be from tax office.

same again, she said she was from the ATO and to call back within 3 days

Yes same thing, she left a message saying she was from the Aust tax office and I would need to provide personal details when I called back. But I couldn't find this number anywhere on the ATO web site.

I just got voicemail and when I call it back no one answer it.

its a fraud call number looking for personal details.

This looks like a fraud call number. The people are claiming to be Australian Taxation office. and asking for personal details.