More credit card knives turning up

They're small, potentially deadly and highly illegal - and more of them are turning up.

Federal Justice Minister Michael Keenan says federal police at airports around Australia have seized 81 so-called credit card knives since October last year - 22 in September alone.

Credit card knives, often bought online, are small knives hidden inside a plastic credit card-sized housing.

Mr Keenan says they are not toys.

"They are a dangerous weapon, they are illegal and they are banned from being taken on domestic and international flights," he said in a statement.

Every state and territory outlaws possession of concealed knives without a reasonable excuse and can impose heavy fines or jail.

Specifically banned are knives manufactured in a manner to disguise or conceal the blade and give the impression of an innocuous item.

"Anyone carrying these weapons should consider themselves warned that you will be detected and dealt with by authorities," he said.