But now Telstra and Internode have said they would not take part in the trials. iiNet has said it would take part only to prove to the Government that its plan would not work, while Optus will test a heavily cut-down filtering model. The Government plans to introduce a two-tiered censorship system of filtering from the ISPs' end. The first tier would be compulsory for all Australians and would block all "illegal material", as determined by a blacklist of 10,000 sites administered by ACMA.

The second tier, which is optional, would filter out content deemed inappropriate for children, such as pornography. Experts say this second tier will have the most marked effect on network performance because every piece of traffic handled by the ISP will need to be analysed for "inappropriate" content. Optus confirmed it would start a live pilot early next year but stressed it would test only the first tier and even then it would only block the current ACMA blacklist of 1300 URLs, as opposed to the Government's expanded 10,000 URL list. Details are scant but the trial will operate in a specific geographical area and customers will be given the option to opt out.

Senator Conroy's office could not explain why it was telling people that the trials would not involve actual customers, which would give little indication of the real-world impact of the filtering plan. Senator Conroy himself has consistently dodged questions about his policy in Parliament.

"How on earth could you conduct a 'live' trial if there are no customers to assess?" Opposition communications spokesman Nick Minchin said. "The minister also continues to be deliberately vague and cryptic about the definition of unwanted content and now he is unable to clarify how this so-called live trial will be conducted, even though he wants it to start before December 24." The Greens today called on the Government to abandon its internet filtering trial, saying it was flawed and doomed to failure.

The plan is opposed by the Greens, Opposition, the internet industry, some child welfare advocates, consumers and online rights groups. They fear the blacklist will be expanded to include the blocking of regular pornography, political views, gambling and pro-abortion sites. "This trial is simply all show. It won't give any meaningful indication of how mandatory internet filtering would work in practice," Senator Ludlam said.

Colin Jacobs, vice-chairman of Electronic Frontiers Australia, said yesterday's incident in Britain, in which virtually the entire country was unable to edit Wikipedia because the country's Internet Watch Foundation had blacklisted a single image on the site, illustrated the pitfalls of mandatory ISP filtering. Senator Conroy has said that, under his filtering plan, Australia would sign up to the same IWF blacklist. "In Australia, not only would the Government have the ability to secretly add any site to our blacklist, but an unaccountable foreign-based organisation would as well," Mr Jacobs said.

"Given that the traffickers of genuine abuse material will not let themselves be slowed down by a filter and are already covering their tracks, the net result that will be achieved here is exactly this: inconvenience, chaos and expense with absolutely no dividend for the children." Senator Ludlam said in a phone interview he believed Labor would drop the mandatory filtering policy in the new year once the now scaled-back trials were completed.

He said the Government could not abandon it now "without losing significant political face". This Saturday anti-censorship protesters are planning to picket in Australia's capital cities, including Sydney's Town Hall and Melbourne's State Library.