Shell Oil says 1,400 of its customers in New Zealand and 4,500 in Australia have had personal details stolen by online hackers.
The company says it first became aware a month ago of the theft of information on customers who had made online applications for a fuel card.
Shell says its system was hacked into, and information including names, addresses and bank account details may have been stolen.
Spokesperson Jackie Maitland says all affected customers have been notified and the company has had no indication that the information has been used.
She says Shell is advising customers to keep a close eye on their bank accounts and to take care if they receive unsolicited phone calls.
Ms Maitland says New Zealand and Australian police are investigating. Shell has closed its online application service until further notice.