The National Party has lost its latest court bid to stop the Engineering, Printing and Manufacturing Union from registering to campaign as a third party in this year's election.
Those involved in the administration of a party's affairs are not legally able to register as third parties.
National's lawyers told the High Court the commission had been wrong in deciding the EPMU did not have such a relationship with the Labour Party.
However, Justice Mackenzie on Thursday ruled the commission properly examined the facts and made no legal error in allowing the union to register as a third party.
He says the Court's role is simply to ensure that the Commission has looked at all relevant matters, not to substitute its own decision.