The Australian Securities and Investments Commission (Asic) is urging the country's online traders to "urgently review" their account security after a wave of hacks.
The watchdog says that during its regular surveillance it has "become aware of several stockbroking account intrusions involving unauthorised access and trading".
Asic is in touch with online broking firms to help affected customers and working with other authorities to find the source, already "pursuing a line of enquiry consistent with similar incidents in overseas markets".
In the meantime, anyone with an online account should check their transaction history and bank accounts, change passwords and make sure their anti-virus software is up-to-date. If any unauthorised trading is spotted, customers need to go to their stockbroker.