THE productivity benefits of high-speed internet access may be a myth, says a New Zealand study that undermines part of the Federal Government's justification for the $43 billion national broadband network.

The study found that while there were economic benefits in having ADSL rather than dial-up, there was little extra value in faster forms such as fibre-optic cable.

Motu Economic and Public Policy Research mapped data from a 2006 study on more than 6000 firms' internet services against administrative tax and employment data to measure productivity. It found those firms that took up the kind of slower broadband services that are readily available in Australia achieved a 10 per cent productivity boost by using it to enter new export markets and buy goods and services online, but there was ''no discernible additional effect'' gained from a faster service.

''Slow broadband'' was classed as ADSL offering 3 megabits a second, and fast as mostly cable services offering 8 megabits a second. Typical speeds in Australia are now at the mid-single digits.

Ari Sharp