Users of the carrier reported issues affecting mobile data services from late afternoon, beginning with intermittent connections before some reported total data outages.
Problem reports sighted by our sister site iTnews revealed at least two core Cisco routers for Optus' data access network went down at the telco's racks in the Global Switch data centre, affecting thousands of postpaid, prepaid and femtocell data access services for at least 90 minutes.
Access to premium SMS messages and Optus' Zoo portal, as well as Virgin and other MVNO customers, were also affected.
Though postpaid services seem to have largely affected customers in NSW and Queensland, prepaid services across Australia were also intermittently affected.
Optus network reseller Amaysim told customers that all 2G and 3G connectivity was down for customers in NSW and Queensland.
"This means all data including mobile broadband is currently not available. You are still able to make and receive phone calls and SMS," it said.
"Apologies for any inconvenience caused, we hope to have this resolved as soon as possible."
An Optus spokesman confirmed 3G data services were affected between 5.45pm and 7.15pm on Monday night but could not say what time all services were restored at time of writing.
Optus' official Twitter account said engineers were working to fix the issue as soon as possible, as of 7.30pm.
Sources also told of some five major incidents at about 8pm, despite most services being restored at that time.