Tens of thousands of Optus customers have had their mobile phone numbers and addresses mistakenly published online.

Optus has told nearly 50,000 customers that their name, address, mobile and home phone numbers were wrongly published in the White Pages, run by Sensis, against the customers' wishes.

The data breach was discovered by Optus during a routine audit of 10 million customers.

The letter Optus sent to affected customers.

About 40,000 of those people were new customers whose details were already breached, according to Optus.