Internet service providers will soon begin blocking access to hundreds of websites that are on a secret blacklist compiled by the Department of Internal Affairs, but critics say the system lacks transparency.

The department this week announced its new Digital Child Exploitation Filtering System, which it said would help fight child sex abuse. The $150,000 software will be provided free of charge to ISPs in a couple of months and will reroute all site requests to Government-owned servers. The software, called Whitebox, compares users' site requests with a list of banned links. If a match is found, the request is denied. It will not cover email, file sharing or borderline material.

Internal Affairs Secretary Keith Manch said the scheme was voluntary for internet service providers, but Yahoo!, Xtra, TelstraClear and Vodafone - representing over 93 per cent of the market - had all expressed interest in adopting it.

Internal Affairs first trialled the scheme in 2007 and 2008 with some ISPs, but IT Minister Steven Joyce said in March that the Government had no plans to introduce internet filtering technology. The minister's office yesterday declined to comment.

Critics say the system has been introduced by stealth and lacks accountability. The department will not disclose the 7000 objectionable websites for fear "inevitably some people would visit them in the interim", effectively facilitating further offending and making the department party to the further exploitation of children.

Internal Affairs censorship compliance head Steve O'Brien said the blacklist would be personally reviewed by staff each month and would be restricted to paedophilic content only.

But systems administrator and IT blogger Thomas Beagle said the system had been deliberately kept "under the radar" to avoid public debate.

Filtering systems in Australia, Denmark and Britain have been accused of serious flaws, with unexplained blacklistings of straight and gay pornography, Wikipedia articles and small businesses.

Mr Beagle said he favoured providing optional clean feeds for users, but believed Governments would be tempted to expand the blacklist in reaction to events.

If the blacklist was managed in an open manner people would be able to challenge what was being done to "protect" them, he said.

Internet NZ said it could be abused and anything that attempted to redirect internet traffic had the potential to "break" the internet.