Senator Conroy has since last year's election victory remained tight-lipped on the specifics of his $44.2 million policy but, grilled by a Senate Estimates committee this week, he said the Government was looking at forcing ISPs to implement a two-tiered filtering system. The first tier, which internet users would not be able to opt out of, would block all "illegal material". Senator Conroy has previously said Australians would be able to opt out of any filters to obtain "uncensored access to the internet".

The second tier, which is optional, would filter out content deemed inappropriate for children, such as pornography. But neither filter tier will be capable of censoring content obtained over peer-to-peer file sharing networks, which account for an estimated 60 per cent of internet traffic. Senator Conroy said Britain, Sweden, Canada and New Zealand had all implemented similar filtering systems. However, in all cases, participation by ISPs was optional and the filtering was limited in scope to predominantly child pornography.

Colin Jacobs, chair of the online users' lobby group Electronic Frontiers Australia said: "I'm not exaggerating when I say that this model involves more technical interference in the internet infrastructure than what is attempted in Iran, one of the most repressive and regressive censorship regimes in the world." Critics of the ISP-level filtering plan say software filters installed by the user on their PC, which are already provided by the government for free at netalert.gov.au, are more than adequate.

Mark Newton, an engineer at Internode, has heavily criticised the Government and its filtering policy on the Whirlpool broadband community forum, going as far as saying it would enable child abuse. He said the plan would inevitably result in significant false positives and degrade internet speeds tremendously. Those views were subsequently widely reported by technology media and blogs. Although Newton identified himself as an employee of Internode - as Whirlpool's rules stipulate - he always maintained his views were personal opinions and not necessarily shared by the company.

On Tuesday, a policy advisor for Senator Conroy, Belinda Dennett, wrote an email to Internet Industry Association (IIA) board member Carolyn Dalton in an attempt to pressure Newton into reining in his dissent. "In your capacity as a board member of the IIA I would like to express my serious concern that a IIA member would be sending out this sort of message. I have also advised [IIA chief executive] Peter Coroneos of my disappointment in this sort of irresponsible behaviour ," the email, read.

It is understood the email was accompanied by a phone call demanding that the message be passed on to senior Internode management. Newton said he found the bullying "outrageous" and Senator Conroy was "misusing his influence as a Commonwealth Minister to intimidate a private dissenting citizen into silencing his political views". A spokesman for Senator Conroy said Newton's accusation that the Government was promoting child abuse was "disappointing and irresponsible". He said the purpose of the email was "to establish whether Mr Newton's views were consistent with the IIA position".

Ironically, Senator Conroy has himself accused critics of his filtering policy of supporting child pornography - including Greens Senator Scott Ludlam in Senate Estimates this week. ACMA released a report in July detailing the results of laboratory tests of six unnamed ISP-level filters.

Only one of the filters tested resulted in an acceptable speed reduction of 2 per cent or less. The others caused drops in speed between 21 per cent and 86 per cent. The tests showed the more accurate the filtering, the bigger the impact on network performance. However, none of the filters were completely accurate. They allowed access to between 2 per cent and 13 per cent of material that should have been blocked, and wrongly blocked between 1.3 per cent and 7.8 per cent of websites that should have been allowed.

"Why would you want to damage the performance and utility of the internet and not actually keep the bad stuff out anyway," said John Lindsay, carrier relations manager at Internode. In Senate Estimates, Senator Ludlam expressed concern that all sorts of politically-sensitive material could be added to the block list and otherwise legitimate sites - for example, YouTube - could be rendered inaccessible based on content published by users.

"The black list ... 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," he said. Despite this, the Government - which distanced itself from the tests by saying they were initiated by the previous government - is pressing ahead with live trials of the filtering system and will shortly seek expressions of interest from ISPs keen to participate.