More than 150,000 Telstra mobile broadband customers may be eligible for a refund, as the telco admitted it failed to send them data usage warnings.
The issue, according to the Australian Communications and Media Authority (ACMA), was that warning messages were sent by SMS to accounts not connected to a mobile phone to receive them.
This resulted in many customers going over their monthly data limit, and paying extra usage charges.
The problem occurred for almost six years, between September 2013 and May this year. Telstra self-reported to ACMA that it failed to send warnings over email, so that all customers could see them.
"Consumers have a right to make informed decisions about their broadband use, and by failing to send these notifications in the correct format people have had to pay excess usage charges without sufficient warning," said ACMA chair Nerida O'Loughlin.
"The [Telecommunications Consumer Protections Code] requires suppliers to send notifications within 48 hours of customers reaching specified data usage quotas on post-paid internet plans."
Telstra has already automatically refunded customers who have complained about excess usage charges or were slugged excess fees of $5 or less.
The company has written to other customers, who may claim a refund through an online form.
"We encourage current or former Telstra mobile broadband customers to contact Telstra for a refund if they believe they incurred excess usage charges as a result of not receiving a usage notification," said O'Loughlin.
More "serious" regulatory action taken on Telstra if any further breaches of the code occurred, according to O'Loughlin.
"The ACMA has asked Telstra to report regularly on its progress of refunding affected customers until all customers have been reimbursed."
The latest mistake comes after Telstra last week announced that it would review 180,000 NBN accounts to see if those customers are owed a refund for underperformance.
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