Communications giant Vodafone has fired several staff after a security breach involving customers' details.
CEO Nigel Dews said the personal details of subscribers could not easily be accessed on the internet as a result of the breach.
But Vodafone has contacted the NSW Police and launched an independent investigation.
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Vodafone did not say how many employees were sacked over the incident.
"We take data security and the storage of our customers' information extremely seriously," Mr Dews said in a statement.
"We are conducting a thorough investigation of the incident and of our own security systems and processes and have taken immediate action."
He said security can "always be improved", vowing measures implemented as a result of the review will "further limit the risk of people doing the wrong thing".
"Some of the initiatives we had already planned for this year are being brought forward, and we will also be conducting an additional independent security review," Mr Dews added.
He dismissed reports that private information, such as home address and credit card details, of millions of Vodafone subscribers could be accessed online.
"Vodafone is also reiterating that contrary to some reports, customer records are not publicly available or stored on the internet, and credit card details held in our database are securely protected," he added.
The company says it has been liaising with the Australian Privacy Commissioner and other government bodies and will co-operate fully with the NSW Police in conducting its investigation.
