In February, the Federal Court hit Optus with a $10 million fine for its treatment of customers who unwittingly bought content through this billing service.

The Australian Competition and Consumer Commission, which initiated the action, said at the time that Optus had admitted misleading consumers and breaching the ASIC Act by billing customers for content produced by third parties which they purchased by mistake through Optus' direct carrier billing service.

The $10 million was to be in addition to the $21 million that Optus and a number of third-party providers had repaid to 240,000 customers before the court action.

The ACCC said under the refund program, Optus had offered refunds to more than 390,000 customers, but thus far only about a quarter had taken up refunds worth $6.7 million.

“Optus committed to providing these refunds, and will continue to contact customers over the coming months,” ACCC commissioner Sarah Court said in a statement on Wednesday.

“Many of the affected customers were charged for content that they never wanted and never used, and from which they found difficult to unsubscribe. In some cases children unwittingly incurred charges.”

The consumer watchdog advised Optus customers to contact 1800 672 501 if they believed they could be entitled to a refund and had not yet been contacted.