Scammers pretending to represent the Australian Taxation Office have stolen more than $800,000 in the past month after frightening people into handing over their money.
The thieves use software that resembles a legitimate phone number to disguise their identities before threatening people with jail or deportation.
The ATO received more than 37,000 reports of scam attempts in November, with one elderly person losing more than $236,000 over five months.
The ATO regularly contacts people by phone, email and SMS.
But Assistant Commissioner Kath Anderson said there were some tell-tale signs it isn't the tax office.
"You can be confident that if there is a number displayed in your caller ID it isn't the ATO," Ms Anderson said on Wednesday morning.
"Taxpayers should be wary of any phone call, text message, email or letter about a tax refund or debt, especially if you weren't expecting it."
The ATO never requests payment of debts via iTunes, pre-paid visa cards or cryptocurrency, and will not request a fee to release a refund.
The tax office only takes direct credits to bank accounts with the BSB numbers 092-009 and 093-003.
It does not send people emails or text messages asking them to click on a link to provide personal details or to download a file or open an attachment.
"If you suspect that you have been contacted by a scammer, you should contact our call centre. It's OK to hang up and phone us on 1800 008 540 to check if the call was legitimate or to report a scam," Ms Anderson said.