Internet Service Providers (ISPs), IT managers and the Electronic Frontiers Australia (EFA) have slammed the federal government's national content filtering scheme and dubbed it a technically impossible token gesture.

The opt-out plan, announced this month by Communications Minister Stephen Conroy, requires all ISPs to filter "objectionable material" from Internet traffic according to a blacklist defined by the Australian Communications and Media Authority (ACMA).

Industry professionals joined the EFA and rebutted the scheme, claiming it is technically impossible and economically infeasible to implement, police and maintain ISP-level content filtering.

According to respondents, such content filtering could turn into an infringement on freedom of information and political activism, and become a moral arbiter for inappropriate content.

EFA chair Dale Clapperton said the proposal is too vague and could result in the censorship of other content such as euthanasia, drugs and protest.

"Even if the system targets child porn, it won't stay that way for long; we are seeing the thin edge of the wedge," Clapperton said.

Clapperton hit back at claims that censorship of drugs, political dissient and other legal freedoms is hysteria.

"Once the public has allowed the system to be established, it is much easier to block other material," he said.

Author of NetAlarmed.com, a parody site of the government's Internet filtering legislation, and Web production manager Michael Meloni said the scheme is a political ploy which lacks transparency.

"All existing reports into Internet content filtering have said it is economically disastrous and impossible to control," Meloni said.

"There are so many things wrong with this I don't know where to start. Mr Conroy has made himself look like a fool who doesn't know how big the Internet is.

"I have zero doubt that it will be ineffective."

A 2003 Howard government-commissioned report on the viability of Internet content filtering stated that government mandated filtering by ISPs will stifle innovation, inflate Internet access prices and cause online usage to plummet.

"We saw this in the early days of the Internet where the walled garden services of the likes of AOL and CompuServe failed to survive the greater desire of users to explore beyond the safe and prescribed content available," the report states.

"Australian ISPs interviewed as part of this project have indicated they will pass on additional costs to their customers. There will also be enforcement costs."

The report rated self regulation as the most economically viable solution over any form of government intervention.

Meloni compared the idea to China's Internet censorship where its filters are overrun by mass generation of new content, and a lack of online police.

"No more than 5 percent, probably far less, of illegal content would be trapped and in addition to the illegal content, there are thousands of porn pages automatically generated every day," he said.

Speaking to media in the speech announcing the legislation, Conroy said the government "makes no apologies to those that argue that any regulation of the Internet is like going down the Chinese road".