The Court of Appeal has released its reasons for dismissing Telecom's appeal against a $12 million penalty for breaching the Commerce Act.
The High Court fined Telecom in April last year for charging its competitors disproportionately high prices to use its network between 2001 and 2004.
The Appeal Court dismissed an appeal against the penalty in June this year, saying the fine was in no way excessive.
The court says Telecom deliberately used its dominant market position to stifle competition for several years and the extent to which it breached the Commerce Act was even worse than the High Court originally found.
Telecom says it is still deciding whether to appeal to the Supreme Court.
The Telecommunications Users Association says the fine imposed on Telecom is a hit the company can cope with easily.
Chief executive of the Telecommunication Users Association Paul Brislen says the case shows the need for a strong regulator.