Australia plans to introduce an internet filtering system to block obscene and crime-linked websites despite concerns it will curtail freedoms and won't completely work.

Adopting a mandatory screening system would make Australia one of the strictest internet regulators among the world's democracies. Authoritarian regimes commonly impose controls. China drew international criticism earlier this year with plans to install filtering software on all PCs sold in the country.

The government said Tuesday it will introduce legislation next year for the filter system to help protect Australians, especially children, from harmful material on the internet. Critics say it will not prevent determined users from sharing such content, and could lead to unwarranted censorship by overzealous officials.

Communication Minister Stephen Conroy said the government would be transparent in compiling its blacklist of websites, but did not give details.

Conroy said the Australian filter was among a number of new measures aimed at strengthening online protection for families. It aims to block material such as child pornography, bestiality, rape and other sexual violence, along with detailed instructions about committing crimes or using illicit drugs.

Such material is already banned from publication on Australian sites, but the government currently has no control over it being accessed on servers overseas.

Conroy conceded it may not be completely successful.

"The government has always maintained there is no silver bullet solution to cyber-safety," he said in a statement. But, "it is important that all Australians, particularly young children, are protected from this material."

Critics say illegal material such as child pornography is often traded on peer-to-peer networks or chats, which would not be covered by the filter.

"The government knows this plan will not help Australian kids, nor will it aid in the policing of prohibited material," said Colin Jacobs, vice chairman of Electronic Frontiers Australia, a nonprofit group that seeks to promote online freedoms.

"Given the problems in maintaining a secret blacklist and deciding what goes on it, we're at a loss to explain the minister's enthusiasm for this proposal," Jacobs said in an online posting.

The group is concerned the blacklist of sites to be blocked by the filter and the reasons for doing so would be kept secret, opening the possibility that legitimate sites might be censored.

Test shows speeds not slowed

Conroy's announcement coincided with the release of a report on a trial that found internet service providers were able to block a list of more than 1,300 sites selected by the government without significantly hampering download speeds.

Telstra, Australia's largest internet service provider, said blacklisting offensive sites using a filter system was feasible as long as the list was limited to a defined number of web addresses, but that no single measure would make the internet 100 per cent safe.

"The blocking of a blacklist of sites is one element of the multifaceted approach that is required to create a safer online environment," Telstra Director of Public Policy David Quilty said.

Jacobs said smaller internet service providers would likely struggle to pay the costs of imposing the new filters. Conroy said the government would help providers implement the filters, without going into details.

The filter would not likely be in place before early in 2011.

Countries such as Egypt and Iran impose strict internet controls, and bloggers have been imprisoned. China has a pervasive filtering system.

Controls in democracies that value free speech are less strict, though internet providers have at times blocked or taken down content deemed to be offensive.

Canada, Sweden and Britain have filters, but they are voluntary. In the United States, Pennsylvania briefly imposed requirements for service providers to block child pornography sites, but a federal court struck down the law because the filters also blocked legitimate sites.