Spark has refunded around $15 million to nearly 113,000 customers after they were charged for a wire maintenance service between 2014 and early 2021.
The Commission began investigating Spark after a complaint in 2020.
Commission chair Anna Rawlings said Spark's conduct likely breached the Fair Trading Act, with the company creating an impression through its website that a wire maintenance service was suitable for all connection types, when it was not.
Spark also made misleading representations through invoicing customers for the service, which had no use or benefit, she said.
"Customers must be able to rely on information businesses provide to them when they are buying, contracting or being invoiced for services.
"Businesses should not be charging customers for services that they don't need or have no use for, or services that they cannot practically benefit from. If extra services or costs are charged, businesses must ensure that information about them is clearly disclosed to customers and is accurate, complete and easy to understand."
Spark had since stopped selling the service to wireless and fibre connect services, the Commission said.
As of last month, Spark had just under 5000 customers left to refund, with 1200 former customers eligible to receive more than a $100 each.
The Commission said for affected customers who were still Spark customers, refunds were applied as credit to their accounts.
Customers no longer with the provider could have the refunds sent to a nominated bank account, donated or transferred to another customer's Spark account.
