Police have smashed a Romanian organised crime gang that allegedly hacked into the computer systems of small businesses, with credit card details of 30,000 Australians used in $30 million worth of illegal transactions around the world.

An Australian Federal Police and Romanian National Police investigation led to the arrest of 16 gang members this week, and seven people in Romania have been charged.

It is said to be the biggest data theft investigation in Australia's history.

Computer hackers allegedly got access to the systems of up to 100 Australian small businesses, where the credit card details of about 500,000 people were stored.