Telecommunications company Optus has confirmed that customer information has been compromised and it's launched an investigation into the breach.
The telco says it immediately shut down the cyberattack and notified Australian Federal Police.
Chief executive Kelly Bayer Rosmarin says she is devastated and sorry.
In a statement, the office for Cyber Security Minister Clare O'Neil says technical assistance is being provided and Australians need to strengthen their protections from online threats.
But the Opposition says the government needs to answer questions.
Liberal leader Peter Dutton says this possibly the largest data breach in Australia's history.
The breach relates to past and current customer data from as far back as 2017.
It includes names, dates of birth, phone numbers and email addresses.
Some customers have also had sensitive details exposed like their home address, as well as licence and passport numbers.
Customers are being urged to be monitor for any unusual activity relating to their bank accounts or notifications that their personal information has been changed.
