Australian financial crime watchdog has issued a "warning" to cryptocurrency ATM operators, warning that many such machines may be helping criminals launder money or defraud victims.

The Australian Transaction Reports and Analysis Centre (AUSTRAC) said in a statement released on Monday that a cryptocurrency working group established in December last year had identified "concerning trends and suspicious activity indicators" related to cryptocurrency ATMs, including links to fraud and scams.

Cryptocurrency ATMs (also known as self-service machines) are physical machines that allow users to buy or sell cryptocurrencies such as bitcoin with cash or a bank card, usually without the need for identity checks like traditional financial services.