Optus will return nearly $9m to hundreds of thousands of mobile phone customers who were billed for a service they didn't request.
Over a period stretching more than two years, the telco charged around 235,000 postpaid mobile and small business customers for SurePage, despite the customers not asking for it.
The service is a voicemail alternative, which diverts unanswered calls to an operator, who relays a message via SMS.
Users are charged $1.20 each time the operator takes a call, according to Optus's website.
The telco blamed the mistake on a software coding error in its billing system, which was fixed in September.
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"Optus apologises to all customers who have been affected by this mistake," said Optus CEO, Consumer Australia, Kevin Russell, in a statement on Friday.
The telco said it will provide a full refund to affected customers.
An Optus spokeswoman says about 30,000 customers will receive a refund of more than $100.
A further 30,000 will receive $50-$100, with the remaining 175,000 receiving under $50, she said.
The telco is anticipating an overall cost of $8.8 million.
Affected customers will receive a letter in the coming weeks.
