Senator Conroy has argued that the tight filter is needed to keep out content that has "no place in civilised society".

Labor senator Kate Lundy has argued for the inclusion of an "opt out" mechanism to give parents and web surfers more discretion over content and hoped to amend the mandatory filter plan when the legislation is considered by Labor caucus soon.

But a spokeswoman for Senator Conroy told The Australian yesterday the filter's purpose was to keep out obscene and potentially harmful materials, such as instructions on how to commit a terrorist act.

The legislation for the mandatory filtering of refused classification (RC) content is scheduled to be introduced during the autumn sittings.

"RC-rated content includes child sexual abuse material; bestiality; sexual violence including rape; detailed instruction in crime, including suicide-related material; violence or drug use; and/or material that advocates the doing of a terrorist act," the spokeswoman said.

"RC-rated content is not available in newsagencies, it is not on library shelves, you cannot watch it on a DVD or at the cinema and it is not shown on television. The government believes this content has no place in a civilised society."

But Senator Lundy told The Australian she thought there was "a lot of interest" in her proposal.

"The feedback I'm getting back from colleagues is that there are concerns around freedom of speech and lack of parental empowerment," she said.

Senator Lundy argued that the filter took control away from parents.

"It doesn't empower parents with the control over what their children are getting," she said. "Senator Conroy's policy just puts in place a blanket filter."

Senator Lundy's proposal will allow web surfers to set the filter at a level they consider appropriate for their household.

Opposition communications spokesman Tony Smith had concerns about the effectiveness of the government filter in protecting children.

"We are yet to be convinced that Labor's mandatory filtering plans will actually be effective or achieve the best results," he said.