Results of the trials are due to be published in July but, in response to a freedom of information request, the Government has admitted that "there are not success criteria as such". "This exposes a major shortcoming in the Government's approach," Opposition communications spokesman Nick Minchin said.

Greens senator Scott Ludlam said: "It sounds as though we'll filter as many sites as the technology allows us to ... that's the reason I think people are so concerned about this in that it seems to be really open-ended." EFA spokesman Colin Jacobs said: "The pilot seems to have been a political exercise in deflecting criticism. Without any benchmarks, the Government can claim it was a success regardless of the cost or performance issues that ISPs encounter." ISP engineer and filtering critic Mark Newton said: "If I spent several hundred thousand dollars on a technology trial at work without having any idea about what the trial was attempting to test, I'd probably be out of a job."

The ACMA blacklist of prohibited URLs, which forms the basis of the Government's censorship policy, contained 977 web addresses as at April 30, according to ACMA. The Government initially planned to censor the entire blacklist but, after widespread complaints that the list included a slew of legal R18+ and X18+ sites, the Communications Minister, Stephen Conroy, backtracked, saying he would only block the "refused classification" (RC) portion of the blacklist.

According to ACMA, 51 per cent of the blacklist, or 499 URLs, is RC content. Based on the Government's budgeting of $44.5 million to implement the filtering scheme, this means the policy will cost $90,000 per URL. "For the cost involved with implementing a mandatory filter, you could perhaps get far more powerful results in relation to striking at the actual heart of the problem, by increasing law enforcement resources to assist with the actual targeting of the producers and distributors of abhorrent content," Senator Minchin said.

Although the new Government plan to block just RC content will not prevent adults from surfing for porn, it is still fraught with difficulty as the RC category includes not just child pornography but anti-abortion sites, fetish sites and sites containing pro-euthanasia material such as The Peaceful Pill Handbook by Dr Philip Nitschke. Sites added to the blacklist in error were also classified as RC, such as one containing PG-rated photographs by Bill Henson.

Senator Ludlam is concerned that the Government is testing an expanded blacklist of 10,000 sites - a 20-fold increase on what it intends to block - despite having not defined the objectives of the policy. He said that, if the objective was to stop the flow of child porn, filtering sites on a blacklist would not achieve this goal because the blacklist would never capture even close to all of the child-porn sites on the web and would be ineffective against peer-to-peer file sharing. "It's like trying to stop the drug trade by blocking one set of traffic lights," Senator Ludlam said.

He noted that this week's arrest of 16 Australian men for accessing a web video of an eight-year-old Russian girl being raped was the result of "people booting down doors, not a net filtering outcome". Senator Conroy's spokesman, Tim Marshall, did not respond to a request for comment.