New Zealand’s internet filtering system went live last month – but the government forgot to mention this to its electorate until its hand was forced by online freedom campaign, Tech Liberty.

Thomas Beagle, a spokesman for the group, said he was "very disappointed that the filter is now running" and that its launch had been conducted in such a "stealthy mode". He added: "It's a sad day for the New Zealand internet."

In an interview with Computerworld this week, he claimed that the filter had gone live on February 1 but the Department of Internal Affairs (DIA) delayed announcing this at until it had met with its Independent Reference Group.

The manager of the DIA’s Censorship Compliance Unit, Steve O'Brien, denied that there had been any subterfuge. The system has been undergoing trials for two years and the media have been aware of this throughout.

He said: "The Independent Reference Group has met and the filter system processes were demonstrated as set out in the code of practice, that is that the website filtering system prevents access to known websites containing images of child sexual abuse."

While the DIA continues to be coy about exactly which ISPs are joining the filter, Tech Liberty understands that Watchdog and MaxNet have already signed-up to deploy the filter system, and that ISPs Telstra Clear, Telecom and Vodafone have said they will do so.

Orcon, Slingshot and Natcom have said that they won't or, in the case of Orcon, that more data is needed as to how the filter will impact customer service.

ISPs also appear to take differing views of what – and when – to tell their customers. Maxnet appear not to have warned their customers officially that the filter is in place: Vodafone has said it will do so when it goes live.

The system employs the Swedish Netclean Whitebox content filter - website requests are filtered by Border Gateway Protocol against a blacklist held on a central server in the government Censorship Compliance Unit. The list is maintained by the Independent Reference Group which actively reviews banned URLs each month to eliminate false positives.

Bans must be justified and signed off by three "warranted inspectors of publications". Government statistics, published in 2009, suggest that the blacklist includes more than 7,000 URLs featuring child sexual abuse – or over ten times the number of links recognised by the UK’s Internet Watch Foundation. It has been built by the censorship unit since 2005 and is affiliated with Europe’s Cospol Internet Related Child Abusive Material Project.

David Zanetti, technical spokesperson for Tech Liberty, said: "We fear that the filter will reduce the stability of the internet in New Zealand. It is a single point of failure, introduces a new and very tempting target for hackers, and by diverting traffic will cause issues with modern internet applications."

Tech Liberty is also concerned by the expansion of government powers that thefilter represents. It entrenches the principle that the government can set up a new censorship scheme and block material with no reference to existing law. Worse, the filter list stays secret, in contrast to the censorship regime that operates in respect of other media, where the Chief Censor must publish decisions banning offensive material.

Cynics have also noted that the launch of the filter comes not long before the eighth round of the Anti-Counterfeiting Trade Agreement (ACTA) negotiations, due to take place in Wellington, New Zealand in April. ®