Optus will be sending letters to numerous phone subscribers in South Australia's Barossa Valley and Mid-North to apologise for a promotion which quoted incorrect phone rates.

"During December and January Optus sent an information pack to these subscribers which included a rate comparison chart between Telstra and Optus, " Australian Competition and Consumer Deputy Chairman, Mr Allan Asher, said today.

"But many of the rates listed were incorrect. In fact, in some cases the STD rates quoted were just local calls. For example, the cost of a 10 minute Telstra call from Tanunda to Gawler was given as $1.45 when it is actually a flat fee for an untimed local call. "

After consumers complained to the ACCC's Adelaide office, Mr Asher met Optus legal representatives in Sydney last week. Optus has admitted its error which resulted from a failure by staff members to counter-check the rates before commencing the promotion.

Optus has given an undertaking to the ACCC that it would take all necessary steps to correct these and compensate consumers.

Optus is posting a letter to every subscriber who was involved and, where subscribers have saved less than they would have expected, has offered to pay a credit to those people on a subsequent bill.