It's a serious question of whether our governing party knowingly infringed copyright of Eminem's song, Lose Yourself.

The National Party is accused of using a backing track for a 2014 election ad that was too similar to Eminem's musical work.

Yet as lawyers have grappled with musical similarity, the structure of rap music, and the difference between imitation and copying, the case has often threatened to descend into farce.

Court rooms, particularly High Court rooms, are usually quiet, sober places.

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But that was shattered from the first day of the hearing, when Lose Yourself was played at full volume to a room of unsmiling lawyers who listened intently to the track.

Over the course of the two-week hearing Lose Yourself has since been played many times, as well as the Eminem Esque track used for the National Party ad.

That was followed by a comparison between Twinkle Twinkle Little Star and the ABC song. They were played to the court by defence lawyers as evidence of the similarities that are possible between different tracks.

Next came a comparison between The Motels' Total Control, Led Zeppelin's Kashmir, and Lose Yourself.

But behind all of the odd moments is a serious debate over the complex workings of copyright law.

The argument for copyright infringement

Lawyer for plaintiff Eight Mile Style, Garry Williams, said the National Party infringed copyright by using a substantial reproduction of Lose Yourself.

In his opening statements, he called the song the "jewel in the crown of Eminem's musical work".

That's because it not only won an Oscar, but also nabbed two Grammys.

"The licensing of the song has been extremely carefully controlled. Despite many requests, it has only rarely been licensed for advertising purposes," Williams said.

"When licensed, it can command in the millions of dollars. That's how valuable it is."

He said that copyright could cover lyrics, musical composition, and the sound recording itself. That meant it didn't matter if the National Party had only used a highly similar beat.

Williams presented evidence to the court of the chain of events which had led to Eminem Esque being used.

Mocked up ads with Lose Yourself as a backing track has tested well with focus groups, so the campaign had worked on finding a track that sounded similar.

This eventually led them to choosing Eminem Esque, from a bank of songs which were provided for commercial use.

Williams then presented the court with evidence from an email chain between members of the National Party campaign team.

One quote from a National Party representative directly questioned if their backing track was too similar to Lose Yourself.

"How can we be confident that Eminem doesn't think we're ripping him off?

"How do we ensure there is no liability with us?"

The email reply was that the National Party wasn't liable, because they were buying a licenced track.

The Party was told the composer would be liable for any copyright problems.

Williams was scathing after reading from the emails.

"That is just wrong, in law.

"Also, the focus is on not just whether it's an infringement. But whether they can shift liability to someone else."

While arguing for damages, Williams argued that the political use of the music should be considered an aggravating factor.

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".

The defence against copyright infringement

National Party lawyer Greg Arthur has made several arguments against copyright infringement, including that it wasn't possible because Lose Yourself wasn't itself original, that Eminem Esque was obtained in good faith from a licenced vendor, and that the key parts of Lose Yourself weren't used in Eminem Esque.

He said that the beat was only a "building block", and on its own was a "non-original syncopated beat".

He argued that the National Party only wanted a strong beat for their ad, not Eminem's track specifically.

"Defendants accept that the combination of the timbre and rhythm meet the broad similarities, but it's a matter of details to get down to what makes Lose Yourself original.

"Eminem-Esque does not reproduce the substantial part of Lose Yourself, so there was no copyright infringement."

"[The beat] might be what people remember about Lose Yourself.

"But it's those melodic lines that give it the distinctiveness.

"To get that rhythm, it could have been Lose Yourself, it could have been Total Control, it could have been Kashmir.

"There's nothing original about having a steady beat. It attracts originality when you add something to it, that melodic line."

"It's more than just the boom boom boom boom."

Arthur argued that it was "absurd" to think the National Party wanted to use Lose Yourself in order to be associated with the rapper.

"Rightly or wrongly, he's been associated with hate speech, violence."

Arthur told the court that the National Party hadn't wanted any distractions in the lead up to the 2014 election, so if it had thought copyright infringement was a possibility, it wouldn't have forged ahead.

He said they trusted their team of advertising and music professionals, and hadn't had any reason to think there was a risk to using Eminem Esque.

What now?

It's now up to Justice Helen Cull to review the substantial evidence in front of her, and decide where it fits within the complex framework of copyright law.

She's reserved her decision, meaning she will review the evidence in her own time and release the decision at a later date.

Justice Cull told the court she hoped her decision wouldn't be "too many months away".

It's a case of 'it takes as long as it takes', although an estimate has been given to the Herald of a possible two to three months time.

That means the decision on the legality of a 2014 campaign ad may be delivered soon before the 2017 election campaign.