An internet filter designed to block access to child pornography, which is being imposed on Optus and Telstra customers, is "trivial" to bypass, say civil libertarians.

But the Internet Industry Association (IIA), which helped design the filtering scheme for ISPs such as Telstra and Optus, said that just because it could be bypassed did not mean it should not be implemented.

The list of URLs that Telstra began to block at the start of this month and which Optus plans to begin blocking at the end of July is maintained by the International Criminal Police Organisation (Interpol). It aims only to prevent access to sites hosting child pornography.

Communications Minister Stephen Conroy announced in July last year that a number of Australian ISPs had agreed to filter a small list of URLs that contained only child pornography after he announced a review of a broader scheme aimed at blocking access to "refused classification" material - not just child pornography.

Academics, ISP experts, political opponents, the US government and a broad cross-section of community groups had long argued that the plan to block a secret blacklist of "refused classification" web pages for all Australians was fraught with issues, for example, that blocked RC content could include innocuous material. That scheme remains under review.