Telstra's newly acquired Asian telecommunications provider Pacnet has been hacked.
Telstra was made aware of the security breach after finalising the purchase of the Hong-Kong based provider on April 16.
A third party accessed Pacnet's corporate IT network, including emails and administrative systems, about a fortnight before the takeover was finalised.
Pacnet customers affected include government entities and small, medium and large businesses.
Telstra's chief information security officer Mike Burgess says malicious software was uploaded to the network, however it's not clear if any data has been stolen or leaked.
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"We have no evidence that data was taken from the Pacnet corporate network but because of the nature of the breach we believed it was important to inform all customers of this," he said.
Group executive of global enterprise services Brendon Riley said Telstra had taken immediate action to protect the security of the network once it was informed of the breach, including additional monitoring.
"We have had no contact from the perpetrators so we don't know the reason behind it or who was involved," he said.
He said the focus was on informing all customers of the breach as finding who did it would be difficult.
Telstra customer details are safe because there's no connection between Pacnet's network and Telstra's network, he added.
