The letter Optus sent to affected customers.
About 40,000 of those people were new customers whose details were already breached,according to Optus.
"The majority of the affected customers’ details were already listed with Sensis prior to joining Optus,"an Optus spokeswoman said.
Sensis insisted that was not the case,saying:"This is an Optus issue."
Personal details have been scrubbed from the White Pages online directory,but they may remain published in printed copies of the phone directory.
Optus has notified and apologised to all its customers affected in the breach.
"We're getting in touch because we've mistakenly released some of your details to Sensis (White Pages) - your name,the below address and phone number - resulting in them being published,"says the letter.