The Department of Internal Affairs website is now back up and running after it was shut down - only days before it was due to be hacked by international cyber activist group.

The website and some other Internal Affairs websites have been down since the beginning of the week - and many still remain affected. The department says it is continuing to get the sites restored.

The outages come after "hacktivist" group Anonymous promised a "denial of service" attack on the department's websites in protest over Internal Affairs implementation of internet filtering which aims to limit the international trade in images of child sexual abuse.

The department's deputy chief executive Stephen Crombie does not know what caused the problem and said Internal Affairs sites have historically been very stable.

"There are always threats and risks to websites. We have no reason to believe that the problem is linked to any particular threat, or even that it involves any hostile action at all," Mr Crombie said.

"We are working to bring the sites back in a way that gives us maximum information about the problem, and we are working through to find the cause. Whatever the cause, the restored websites will be more resilient than they were, but no website is invulnerable and we cannot guarantee 100 percent availability in perpetuity."

In a video posted by NZAnonOps on Youtube, Anonymous threatened to bring down the department's website next Monday.

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

Anonymous gained notoriety last year for shutting down the websites of 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.