The judge charged with deciding whether the National Party infringed Eminem's copyright, has warned the verdict could take months.

A High Court hearing has been under way in Wellington for the past fortnight, with the National Party accused of using an unauthorised copy of Eminem's Lose Yourself for a 2014 election campaign ad.

Both sides have now presented their evidence and made their closing arguments, with Justice Helen Cull reserving her decision.

She warned that it wouldn't be a speedy verdict.

Advertisement

"I will be reserving my decision, that won't come as a surprise, but I hope it isn't too many months away."

Earlier in the day, lawyers for Eight Mile Style argued that any copyright infringement was made worse by the track being used for political purposes.

Barrister Garry Williams said the National Party knew it was copying Eminem's work, and the risk it was taking.

"Despite this they did not consult legal experts, but proceeded, with their eyes open."

Williams said that and the political use were aggravating factors the judge should consider when looking at any possible damages.

"One of the factors is flagrancy, and that's the avenue by which aggravated and exemplary damages are able to be given.

"But you're also directed to take into account all the benefits the defendant derived by its use.

"The political use, the absence of the strict creative controls the plaintiff would usually have."

Media have been barred from court while specific commercial details of the song licencing were discussed.

But Williams has previously referred to the licencing for Lose Yourself as being worth "in the millions".

Justice Helen Cull pushed Williams to clarify which parts of Lose Yourself would be subject to copyright protection under the law.

"Part of this tension has been from the 17th century definition of music, in which it's seen as the melody," Cull said.

"How relevant is the melody line here, or is it the overall work?"

Williams argued that specific melody notes were not relevant, but rather the overall sound of a song.

"People no longer, when composing music, note it down on paper.

"They play it, record it, and mix it. Where this gets us to is the aural comparison you need to take in a music composition case is much more important than the notes as they appear written down.

"The structure, the tempo, the rhythm, all of that should be taken into account when assessing what a musical work is."

Lose Yourself was released in 2002 as part of the movie 8 Mile.

It won both Oscar and Grammy awards.