The criticisms are contained in submissions to the Department of Communications released yesterday for the first time.

Between them, the companies control more than 80 per cent of online search engine traffic in Australia.

They join ACT Labor senator Kate Lundy, Greens communications spokesman Scott Ludlam and online activist groups Get Up, Electronic Frontiers Australia and Anonymous in a chorus of disapproval for the scheme.

In its 24-page submission, Google whose corporate motto is ''don't be evil'' warned that under mandatory filtering ''the scope of content to be filtered is too wide''.

''We have a bias in favour of people's right to free expression. While we recognise that protecting the free exchange of ideas and information cannot be without some limits, we believe that more information generally means more choice, more freedom and ultimately more power for the individual,'' the company said in its submission.

The submission goes on to say that filtering could give parents a false sense of security, damage Australia's international reputation, could be easily circumvented and that there was a lack of transparency around what would be filtered.

It also warned that mandatory filtering could hit internet speeds a factor played down by federal Broadband Communications and the Digital Economy Minister Stephen Conroy. ''The implementation of a mandatory filtering regime across all of Australia's hundreds of ISPs, millions of internet users, and billions of web pages accessed is a massive undertaking and we remain concerned about the potential impact on user access speeds from filtering,'' the company wrote.

The four-page submission from Yahoo warned the US-based company remained concerned about the proposal but was willing to work with the Government on the plan.

''Yahoo is completely supportive of government efforts to create a safer online environment for children, however, we remained concerned about key elements of the current policy on internet filtering,'' the submission noted. Both companies called for a stronger focus on cyber safety and increased education campaigns as an alternative to mandatory filtering.

For more on this story, see the print edition of today's Canberra Times.