UPDATED: The Department of Internal Affairs said it has no links to suggest the group of hackers known as Anonymous were responsible for several of their sites being unavailable since Tuesday.

As of 2.21pm Thursday, 41 of the Department's websites, including the Departnent of Internal Affairs parent site, have been intermittently down since midday Tuesday, senior communications manager Tony Wallace told NBR today, but the department could not confirm the cause of the outtages.

"We don't know that it's a hacker."

Mr Wallace said the department was not ruling hacker involvement out and if the outtages are proven to be part of Anonymous' promise to attack the department, it will look at ways to better protect the sites. He said the department was aware of the threat, but that threats and risks were a general feature of the web environment.

"We're continually preparing for those."

He said it was important for people to remember that the department merely provided the means for service providers to filter content, but that it was a voluntary filter that was ultimately up to the ISPs to implement. Anonymous have threatened to attack the department for their "Internet censorship", announced in a video on YouTube. In a media release this afternoon the department said the filter was aimed solely at reducing the trade in child sexual abuse and blocked access to sites which contained images, videos or promotions of such abuse. The release stated that there was no proposal for the filter to become compulsory.

Mr Wallace said their focus now was on getting the websites back up and running again, and providing basic information on affected sites to provide alternative contact details.

He confirmed that the civil defence website had been affected. A temporary website is in place with key information, he said.

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Days before hackers were set to attack it, the Internal Affairs website is down.

A spokesman for the department said it had not yet established what had happened but was investigating. Restoring services was the priority, he said.

A video on the internet by hacker collective Anonymous detailed its opposition to Internal Affairs implementing internet filtering this month.

"Internet censorship as seen in China, India, Australia, the United States as well as the United Kingdom has become one of the greatest atrocities to free speech and government transparency since the cold war," the group said.

"It is for this that we the people, must and will step forward to dismantle the Government's control over the internet."

The group promised a series of attacks to start next Monday, but the site was already down today.

"The attacks will continue until The Department of Internal Affairs vetos their own decision and releases the free flow of information to New Zealand."

The message concludes: "You cannot find us. You cannot stop us. We are Anonymous. We are Legion. We do not forgive. We do not forget. Expect us."

A message on the Internal Affairs website said it was temporarily unavailable and apologised for the inconvenience.

Technology writers Kris Notaro and Wes Strong have written about Anonymous saying it began as a movement in 2003 on a series of internet chat boards and has gone from targeting small time hypocrites to large multinational corporations bringing it from the background of hacker culture to the forefront of global politics.

It gained notoriety in 2010 after shutting down Mastercard, Visa, and Paypal during what it called Operation Payback. Those major corporations stopped providing their services to Wikileaks, which had been using them to accept donations into the Wikileaks defence fund.

Last year's Parliament InTheHouse link was taken over by Turkish hacker Iskorpitx.