An estimated half a million Optus Mobile customers were affected by a network outage last night, with only one in three calls reaching intended recipients.

A spokesman for Optus customer service said the outage was due to a problem with Optus' "HLR" (Home Locator Register) system, which routes the calls being made.

At 7:30pm Wednesday the spokesman said the outage occurred "a couple of hours ago" and was unable to provide an indication of when services would be back to normal.

"The outage has affected one sixth of Optus' four million customers," adding that Optus' home network customers were not affected.

Services hadn't been restored by midnight but were operating normally this morning.