The High Court has reserved its decision on whether the Electoral Commission was right to allow the Engineering, Printing and Manufacturing Union to register to campaign as a third party for this year's election.
The National Party sought the judicial review of the decision, which would allow the union to spend up to $120,000 on election advertising.
Legally those involved in the administration of a political party are ineligible to become third parties, and the National Party says the commission took an unduly restrictive definition of what constitutes the affairs of the Labour Party.
In Wellington on Monday, a lawyer for the National Party told the court the commission looked at only the Labour Party rules when making its decision, when it should have also considered the union's rules.
However, the commission told the court it had regard to all relevant matters in reaching its decision, and that National simply wants the court to impose a different view to that formed by the commission.