Telecom has been found guilty of anti-competitive behaviour in the high-speed data transition market.
The High Court in Auckland ruled Telecom abused its substantial market power to prevent, and detered competition in that market between 2001 and 2004.
The court found Telecom charged its competitors disproportionally higher prices for wholesale access to its network.
Telecom says it is disappointed with the ruling and says regulation introduced in 2004 and operational separation mean those particular market conditions no longer exist.
The court is yet to set a penalty.