As such, Telecom had breached section 36 of the Commerce Act.
As a result of the breaches, in April last year the company was hit with a $12m penalty - the highest handed out in history.
Telecom company then headed to the Court of Appeal and mounted a full-scale attack on the original judgement.
But in a ruling made public this afternoon, Justices Susan Glazebrook, Ellen France and Robert Chambers have thrown out Telecom's bid.
"We do not accept Telecom's submission that the High Court erred in concluding that there was direct evidence of anti-competitive purpose," their ruling said.
The company has also been ordered to pay the Commission costs "for a complex appeal".
While Telecom lost its bid, Justices Glazebrook, France and Chambers accepted a cross-appeal from the commission.
This related to a decision from the High Court that it did not have the jurisdiction to rule on Telecom's conduct prior to 2001.
Justice Chambers said he accepted "the Commission's challenge to the High Court's conclusion".
"I would hold that the High Court did have jurisdiction to give declaratory relief with respect to Telecom's pre-2001 conduct," he said.
As such the Court of Appeal amended the declaration against Telecom to say that the company took advantage of its "dominant position/ market power from February 1999 to late 2004...for the purposes of deterring potential or existing competitors in the wholesale market for backbone transmission".
Commerce Commission chairman Mark Berry welcomed the ruling.
"While the same conditions do not exist today, this ruling is important as it reinforces the fact that it is illegal for a business with a substantial degree of market power to take advantage of that power to deter or prevent rival businesses from competing effectively."
Telecom also appealed against the $12m penalty it received, but the three judges said this would be dealt with in a separate judgement.
While the declaration against Telecom now includes its pre-2001 conduct, the Commission has conceded it is not entitled to seek penalties against the company for its earlier breaches