The Government's Classification Board appears to have fallen victim to a prank from opponents of a proposed mandatory internet filter.

Satirical messages comparing the board's actions to that of the Chinese Government appeared on the front page of the website for a few hours last night.

"This site contains information about the boards that have the right to CONTROL YOUR FREEDOMZ," the message said.

The Board is responsible for classifying websites and video games, and its decisions are the basis for the Australian Communication and Media Authority's (ACMA) blacklist of banned websites, the subject of recent controversy when it was purportedly leaked on the internet.

"The Classification Board has the right to not just classify content (the name is an ELABORATE TRICK), but also the right to DECIDE WHAT IS AND ISNT [sic] APPROPRIATE and BAN CONTENT FROM THE PUBLIC," the fake message went on.

"We are part of an ELABORATE DECEPTION from CHINA to CONTROL AND SHEEPIFY the NATION, to PROTECT THE CHILDREN. All opposers must HATE CHILDREN."

A spokeswoman for the board this morning said the body was aware of the incident, and was investigating. She said the investigation would look into who was responsible, and how much of the board's website was affected.

The board's website has been inaccessible for some hours.

The attack appears to be linked to recent criticism of the Government's proposed mandatory internet filter.

The board is responsible for rating websites which then make up the ACMA blacklist, containing websites that would be blocked if a filter was put in place.

Internet advocacy groups say a secret blacklist is an attack on democratic values, and the existence of a blacklist actually makes it easier for people to access the very content supposed to be blocked.

They say this was demonstrated recently with the leaking of what purported to be the blacklist, including links to thousands of websites.

Communications Minister Stephen Conroy last night defended the Government's plans, saying the list has been in place for nine years, and it is based on the same criteria on which television, radio and book content is rated.

He says the internet filter - which is only in a trial phase - will not limit political content as many critics have said.

"If something is refused classification - if it promotes rape, promotes incest, or similar - it makes it onto the blacklist," he said on ABC1's Q and A.

"The Government makes no apologies for that. No political content will be blocked, that is not the intention and in fact the law would have to be changed in order for that to happen.

"It's not going to happen, I cannot be clearer on that."

He also responded to questions about seemingly innocuous websites, like PG photographs by Bill Henson, a Queensland dentist and a boarding kennel, being placed on the blacklist.

He said the Henson website was a "technical error" by an ACMA official, and the other two were the result of Russian mafia infiltrating the websites' servers and planting child pornography there.

Western Australian Greens Senator Scott Ludlum says the recent furore over internet filtering shows how difficult it is for the Government to control the internet with a crude filter.

"At the moment there's still a temptation to treat the internet like any other medium, when it is not," he said.

"It's fundamentally different and I think that's in a way why we've always been in favour of an approach that says for child pornography and that sort of material, the solutions lie around law enforcement, education, allowing parents to set these boundaries in their own homes.

"For this type of material, law enforcement is the key."

He says it is a concern that the presence of seemingly harmless sites on the list was only revealed when the list was published a week ago.