The Federal Government has deferred the introduction of its mandatory internet filtering program.

Communications Minister Senator Stephen Conroy says the filter will not be put in place until an independent review can be carried out into what content would be banned.

The review, which Senator Conroy says is likely to take about a year, will look at what makes up "refused classification" rated content.

Senator Conroy says internet service providers Telstra, Optus and Primus have agreed to block websites known to contain child pornography in the meantime.

"I applaud these industry members for taking this stance, for stepping up to the plate, in recognition that there is some content that is not acceptable in a civil society," he said.

"This approach is consistent with what is happening around the world."

The Government announced the filter two years ago as part of its cyber safety program to protect children from pornography and offensive material. Last year it ran tests on the system.

Senator Conroy had intended to introduce the legislation in the first half of this year, but deferred it to later in 2010.

He says the refused classification rating, according to the national classification scheme, includes child sexual abuse imagery, bestiality, sexual violence, detailed instruction in crime, violence or drug use and material that advocates a terrorist act.

"Under Australia's existing classification regulations this material 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," Senator Conroy said.

"Under laws passed by the previous government, refused classification material is not available on Australian hosted websites.

"Unfortunately where this content is hosted overseas nothing can be done to remove it."

Some of the grey areas of internet censorship include images of crimes taking place, graffiti or stencil art, and what some people would refer to as sexual fetishes.

The plan has been criticised by internet users who claim it will slow download speeds and lead to unwarranted censorship.

Senator Conroy has not yet announced who will conduct the independent review.

SingTel Optus' head of corporate and government affairs, Maha Crishnapillai, says the Attorney-General should decide what should be on the refused classification list.

But he says Optus expects transparency on how the blocked URLs come to be on the list.

"The problem we have is international sites and so the only way we can deal with that is to block those and I suspect all of our customers want us to block child pornography sites," he said.

"One of the reasons we have said we want to step up and offer this voluntarily is that we think there are ways that the industry [can] cooperate to do that without the need for the mandatory legislation. However... if legislation is required we will quite happily work with Government on that."