A spokesman for Senator Xenophon said, should the filtering plan go ahead, he would look to use it to block Australians from accessing overseas online casino sites, which are illegal to run in Australia.

In a Senate Estimates hearing last week, the Communications Minister, Stephen Conroy, confirmed that his proposal would force ISPs to implement a two-tiered filtering system.

The proposed censorship is more advanced than that in any liberal democracy and would put Australia on a par with oppressive regimes such as Iran, the internet industry says. Despite his earlier promises that Australians would be able to opt out of any internet filters, Senator Conroy said the first tier would be compulsory for all Australians and would block all "illegal material", as determined in part by a blacklist administered by the Australian Communications and Media Authority (ACMA). The second tier, which is optional, would filter out content deemed inappropriate for children, such as pornography.

But asked to specify the categories of content that Senator Fielding would like blocked by the mandatory first tier, a Family First spokeswoman indicated the party would want X-rated and refused classification (RC) content banned for everyone, including adults. "Family First would consider a mandatory ISP-based filtering system that protects children by blocking illegal content like child pornography, but allows adults to opt out of filtering to access material classified R18+ or less," Senator Fielding's spokeswoman said.

According to the Broadcasting Services Act 1992, X18+ content includes hardcore pornography, while content that is refused classification includes that which depicts drug use or sexual fetishes. Both are a step above R18+ content, which typically includes adult themes. The online users' lobby group Electronic Frontiers Australia expressed fears that the internet filters could be used as a bargaining chip every time the Government needed to pass a piece of important legislation. "Any group with an axe to grind and political clout will be lobbying the Government to blacklist websites which they object to," EFA spokesman Dale Clapperton said.

"Having all Australians' internet access subject to a secret and unaccountable government blacklist is completely unacceptable in a liberal democracy such as Australia." Clapperton said most adult pornography on the internet was already "prohibited content" under the Act, and pro-euthanasia, pro-anorexia and pro-piracy websites could easily be caught by the system.

Today, such prohibited content, if hosted overseas, is added to ACMA's blacklist but Australians are still able to access it if they wish. This would not be the case if mandatory ISP filtering was introduced. "Senator Conroy talks about blocking access to 'illegal material', but the ACMA blacklist of 'prohibited content' is not limited to material which is illegal - it includes X-rated material, and R-rated material unless it is protected by a government-approved restricted access system," he said. Greens Senator Scott Ludlam expressed similar concerns when grilling Senator Conroy in Senate Estimates last week.

He said all sorts of politically sensitive material could be added to the blacklist and otherwise legitimate sites - for example, YouTube - could be rendered inaccessible based on content published by users. "The blacklist ... can become very grey depending on how expansive the list becomes - euthanasia material, politically related material, material about anorexia. There is a lot of distasteful stuff on the internet," Senator Ludlam said.

John Lindsay, carrier relations manager at Internode, said: "I don't see that what Fielding has just described to you is necessarily any different to what the public should expect from the Government's as yet unstated filtering regime, because we haven't got a clear explanation as to what the Government's actual mandatory blacklist looks like." The Opposition's communication spokesman, Nick Minchin, said it would take "a lot of convincing" for the Coalition to support the Government's filtering plan. "That's the problem with having this sort of highly centralised government-mandated nationwide filtering system," Senator Minchin said in a telephone interview.

"The argy-bargy that would result over what is in and what is out strikes me as being almost impossible to manage and it would be a cat chasing its tail." The Government's own laboratory trials have found that presently available filters are not capable of distinguishing adequately between legal and illegal content and can degrade internet speeds by up to 86 per cent.

Despite this, and significant opposition from ISPs, the Government will soon seek expressions of interest from telcos seeking to be part of a live trial of the filters.