An Optus spokesperson confirmed there was "a minor systems error" that resulted in the letters being sent. On discovering the problem, Optus said it sent SMS notifications to all affected customers asking them to ignore the letters and apologising for sending them.
"No-one will lose their service or have their service changed or affected," the spokesperson said.
The Optus yes Time promotion allows subscribers to call other Optus mobiles between 8pm and midnight with no charge for the first 20 minutes. However, the promotion is subject to a "Fair Go" usage limit of 2000 minutes per mobile per month.
The letter sent by Optus warned customers that they had exceeded the limit. It said that if the limit was exceeded for a second time, customers would be asked to pay for any usage over 2000 minutes. A third breach would see Optus charging "the cost of all yes Time calls to your mobile account for the remainder of the promotion," the letter said.
One September Optus mobile bill examined by iTnews came with a letter saying the subscriber had used just over 2000 yes Time minutes, while the bill itself showed only 30 seconds use.