An Australian government proposal for a mandatory web filter has been criticised by key internet players Google and Yahoo as a heavy-handed measure that could restrict access to legal information.
The government says the aim of the filter is to block access to sites with material that include child pornography, sexual violence and "detailed instruction in crime".
But advocates of free information are concerned that compulsory screening could eventually grow to bar other controversial but otherwise legal material.
The statements from Google and Yahoo were among 174 submissions sent to the Australian Department of Communications as part of a public consultation exercise on the proposed filter.
Stephen Conroy, the Australian communications minister, said the views would be considered before the final draft goes to parliament later this year.
"A range of views have been expressed in the submissions. [We will] examine whether the ideas can be used to enhance the proposed accountability and transparency measures," Conroy said in a statement.
Conroy has previously said he wants the filter introduced to bring the online world in line with censorship standards applied in Australia to material such as films, books and DVDs.
The Australian plan was criticised earlier this month by media rights watchdog Reporters Without Borders (RSF), which said the proposed filter would hurt free speech.
As a result RST placed Australia on its list of countries under surveillance in its annual "Internet Enemies" report on online freedoms.
In its submission released on Wednesday, Google said Australia would become one of the strictest internet regulators among the world's democracies if it adopts the proposal.
The internet search giant said the main concern was that "the scope of content to be filtered is too wide", adding that it already had its own filter to block child pornography.
"Some limits, like child pornography, are obvious. No Australian wants that to be available … and we agree," the Google submission read, also suggesting that the filter would slow browsing speed.
"But moving to a mandatory ISP level filtering regime with a scope that goes well beyond such material is heavy-handed and can raise genuine questions about
restrictions on access to information."
On Wednesday Lucinda Barlow of Google Australia said the proposal went beyond filters used in Germany and Canada, which block child pornography and, in Italy, gambling sites.
"This enters the grey realms of restricted classification, seeking to ban politically and socially controversial material," she told the Australian Broadcasting Corp.
Yahoo's views struck a similar note, saying the filter would block many sites that contain controversial information such as euthanasia discussion forums, safe injection information, or gay and lesbian forums that discuss sexual experiences.
"There is enormous value in this content being available to encourage debate and inform opinion," read its submission.