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.
"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.
