Australian airline Qantas says a data hack on Monday exposed the personal information of six million customers and it expects the amount stolen to be “significant.”

A cyber hacker broke into a database containing the personal information of millions of customers, Qantas said, in Australia's biggest breach in years and a setback for an airline rebuilding trust after a reputational crisis.

Australian airline Qantas said that a hacker made off with a trove of customers’ personal data including passenger names, emails, phone numbers, birth dates and frequent flyer numbers.