SYDNEY - Australia's largest telecoms firm Telstra said on Tuesday it had suffered what it called a small data breach, a disclosure that comes two weeks after main rival Optus was left reeling by a massive cyber attack.

Telstra, which has 18.8 million customer accounts, equivalent to three-quarters of Australia's population, said an intrusion of a third-party organisation exposed some employee data dating back to 2017.

A Telstra internal staff e-mail put the number of affected current and former employees at 30,000, according to local media.

The data that was taken was "very basic in nature", limited to names and e-mail addresses, a company spokesman said in a statement.

"We believe it has been made available now in an attempt to profit from the Optus breach," the spokesman also said without elaborating.

Telstra did not comment on the number of people affected or when the breach occurred, but said it affected current and former staff only.

Australia's telecommunications, financial and government sectors have been on high alert since Optus disclosed on Sept 22 that a breach of its systems may have compromised up to 10 million people's accounts. The data exposed included home addresses, driving licences and passport numbers.

Optus owner Singtel has said that it is assessing the potential cost of the attack, while law firms contemplate class action suits.

The Australian government, which believes the breach was due to a basic security gap, has continued to slam Optus for describing the attack as sophisticated and for delays in updating affected customers.

"Optus senior management are kidding themselves if they want a medal for the way that they have been communicating," Government Services Minister Bill Shorten told reporters on Tuesday.

An Optus spokesman declined to comment on Mr Shorten's remarks, but said the company was working as quickly as possible to provide accurate updates to impacted customers. REUTERS