Mr Abbott hasn't been convinced internet filtering can really trap net nasties as there was no substantial technical evidence.

"We certainly haven't seen the kind of technical assurances that we'd need so let's wait and see how this thing develops," he said in response to a question on ABC TV's Q&A program last night.

"I want to see protections in place. I don't want to see our kids exposed to really terrible stuff on the internet. On the other hand I don't want to see the internet destroyed by a filtering system that won't work so I guess for me it's a factual issue.

"Can you have a filtering system that is effective, that doesn't lull parents into a false sense of security and which doesn't in the process make the internet ineffective as the kind of marvellous research tool and educational device that it is?

"I don't know at the moment ... I just don't know," Mr Abbott said.

His understanding of the scheme is that the filter would only stop access to material that would be a crime to possess -- a view debunked by online and legal experts.

Communications Minister Stephen Conroy, however, argues that his filter is "100 per cent accurate" following a live trial of the blocking process with nine ISPs.

Speaking on ABC Radio last week, he said: "The internet filter that we are proposing has been shown to be 100 per cent accurate - no over blocking, no under blocking. It's 100 per cent accurate because it targets a defined URL address ... it's an individual page within a website."

Joe Hockey is one opposition frontbencher who has made his views on the filter crystal clear.

In his speech "In Defence of Liberty" at the Grattan Institute last month, the shadow treasurer expressed concern the government would take advantage of the filter to broaden its censorship reach.

"The government’s Internet filtering proposals is a scheme that is likely to be unworkable in practice.

"But more perniciously it is a scheme that will create the infrastructure for government censorship on a broader scale.

"Protecting liberty is about protecting freedoms against both known and future threats. Some may argue that we can surely trust a democratically elected government in Australia to never try to introduce more widespread censorship. I am not so sure," Mr Hockey said.

The federal government wants to introduce mandatory ISP filtering so online content rated RC or Refused Classification would be automatically blocked by ISPs.

Legislation to force ISPs to start blocking the internet is expected to be introduced in the second half of this year.

Kevin Rudd is set to cast the online filtering net wider in the coming election, a major backflip from original plans unveiled at the previous polls.

But online experts say the definition of RC is too broad, highly subjective and can trap legal, adult material.

Labor first unveiled its internet filtering plan in the lead-up to the 2007 election and people hardly bat an eyelid since the emphasis was explicitly on the safety of children - not adults - on the internet.

In his manifesto - Labor's Plan for Cyber safety – Senator Conroy, then Opposition spokesman for communications and IT, said: "Labor recognises that cyber safety today is an important part of children's overall health and well-being, yet it is one that is not being adequately addressed by the Howard government.''

Senator Conroy laid-out a five-pronged plan to enhance cyber safety for children, including the introduction of a "clean feed'' to stamp out net nasties.

"(Labor will) provide a mandatory 'clean feed' internet service for all homes, schools and public computers that are used by Australian children.

"ISPs will filter out content that is identified as prohibited by the Australian Communications and Media Authority (ACMA). The ACMA 'blacklist' will be made more comprehensive to ensure that children are protected from harmful and inappropriate online material,'' he said at the time.

Today, Senator Conroy has his sights firmly set on stamping out RC content which he argues includes child sexual abuse imagery, bestiality, sexual violence, detailed instruction in crime, violence or drug use and/or material that advocates the doing of a terrorist act. They are illegal to purchase in the physical world and such laws should extend to the virtual world, he says.

The Classification Board determines what type of content falls under RC and other ratings.

At the moment, there are three content areas that guide the board's decisions - film, games and publications. There is no category for the internet and as such, videos on Google's YouTube site, for example, are judged as films.

"As long as it's moving it's a film. Otherwise it's a publication,'' a Classification Board spokeswoman said.

(A public consultation process is underway to see whether there should be an R18+ classification category for computer games.)

However, Senator Conroy's definition of RC is not exhaustive and can be misleading as online free speech advocate Irene Graham points out in one of many examples.

"RC material is a wide-ranging category of content which includes material deemed to 'offend against the standards of morality, decency and propriety generally accepted by reasonable adults','' Ms Graham said.

The depiction of actual sexual activity between consenting adults involving lawful fetishes could also be classified as RC content and blocked by Senator Conroy's filter, she said.

Senator Conroy's office has admitted that the process is open to one's interpretation.

Asked whether the government believes there are no grey areas when it comes to RC material online, a spokeswoman for Senator Conroy said: "State and federal attorney's-general decide on the guidelines for Refused Classification based on 'community standards'.

"There will always be content that various individuals disagree with but importantly it is the Classification Board, an agency at arm's length from the government and representative of the community, which makes the decisions about individual cases,'' she said.

Cyberspace Law and Policy Centre executive director at the University of New South Wales, David Vaile, says the internet cannot be treated as a tangible product.

"The internet has eroded the distinction between published and unpublished - where this meant central production and distribution, and a sale based revenue model,'' Mr Vaile said.

"Now everyone can and is a publisher, and the volume is so high that humans cannot be afforded to classify it all. This means the classification model has broken for the internet.''

Internet giant Google also believes the filtering regime is too broad. "(It) limits freedom of access to information. A broadly scoped mandatory filter could block important content which informs public debate on socially and politically controversial issues and we do not believe that governments have the right to block that information,'' a Google Australia spokeswoman said.