Anti-porn software developed for schools by a company with links to fundamentalist Christians has been criticised for blocking students' access to leftist political forums and websites on sexuality and health.

Education Minister Trevor Mallard approved the Watchdog Corporation's CampusNet filtering software last year, as part of a $9.5 million package to help schools screen out hackers and objectionable material such as pornography.

But the software - used by 500 schools throughout the country and one of three programs offered to them for free - has been called "subversive censorship" after it barred access to a number of alternative-themed websites, including political forum infoshop.org and current affairs portal gaynz.com, which contains articles on HIV/Aids.

Another user reported being barred from Happy Clapping Homos, a satirical site which mocks fundamentalist Christians, after the software classified it as gay porn.

Mr Mallard is understood to be seeking advice from his officials after a complaint from a Takapuna Grammar student.

The software company, Watchdog Corporation, describes itself as founded on Christian principles and supports organisations such as Christian broadcaster Radio Rhema and City Impact Church.

Watchdog's managing director, Peter Mancer, would not speak to the Weekend Herald, referring all questions to the Ministry of Education.

A ministry spokesman said the company had said it would comply with any school's request to unblock a site.

Watchdog manages a website blacklist. An IT specialist said non-pornographic gay content would not be blocked unless manually added to the list.

Green MP Nandor Tanczos said Mr Mallard should withdraw approval of the software.

Paul Litterick, secretary of the Association of Rationalists and Humanists, called the filtering a form of "subversive censorship".

"They're just taking out the bits [of the internet] that they don't want and not allowing anybody to see them."

Takapuna Grammar is standing by the software. Acting principal Terry Holding said the school regarded Watchdog as a suitable screening program.

"The extent of the internet and the rapid proliferation of exploitive material mean that schools need a first-level system to make some general judgments about access."

Liz Butterfield, director of the Government-funded Internet Safety Group, said some sites could be blocked accidentally.

Schools had the flexibility within their system to unblock sites if they wanted to access them.