The news is an about-face on Senator Conroy's decision to shun Optus from the first round in favour of six other ISPs.
It appears to add considerable weight to the technology pilot, which previously counted Primus as the largest participating ISP.
"The participation of Optus will help ensure the Government obtains robust results from the pilot which will inform the evidence-based development of our ISP filtering policy," Senator Conroy said.
"Optus is an active participant in the cyber-safety space, with a range of initiatives to help Australian families enjoy a safer internet experience. I am pleased to announce its participation in the pilot."
In news that will come as a surprise to many, Conroy also revealed that the Government "is also working constructively with Telstra on the technical testing of ISP filtering technologies".
"These tests do not involve actual customers and therefore are not being conducted as part of the pilot, however it is hoped that the results will feed into the Government's policy considerations," Conroy said.
Telstra BigPond had claimed back in December that it would not participate in the trials due to undisclosed "customer management issues", widely understood to be a fear that users would both soak up limited support resources and blame the trials for any perceived loss in internet speeds.
Another small ISP, Nelson Bay Online, has also joined the trials, it was announced today.
Other participants are Primus Telecommunications, Highway 1, Netforce, OMNIconnect, TECH 2U and Webshield.
"Consultations continue with a number of other ISPs that have applied to take part in the filtering pilot," Conroy added.
Optus has always indicated it would take part in ISP filtering trials, despite being opposed to mandatory filtering.
"Willingness to participate in the trial does not necessarily indicate support of mandatory filtering," the carrier said on its FAQ late last year. "Optus would rather work with Government to legitimately understand the implications of this type of filtering process than have it mandated."
In related news, the Government said it is examining the introduction of ISP-level filtering for Refused Classification material as identified under the National Classification Scheme and the ACMA complaints process.
It is also considering optional ISP content-filtering products for those families who wish to have such a service.