The Rudd Government is continuing to encounter ISP resistance to its plans for a trial of Internet filtering technology, with SA-based national operator Internode and Perth's iiNet refusing to have anything to do with it, and Optus saying it will participate only in a strictly limited way.

Earlier this week Telstra declined to take part in the trial due to what it called "customer management issues".

Optus yesterday said its participation would be "strictly limited" to filtering an ACMA blacklist, and insisted that the test only be run in a specific geographic area, while customers must be able to opt-out.

"Optus is participating in order to accurately gauge the impact that this type of filtering would have on its network, including download speeds and customer experience," a spokesperson said.