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.