The cost to the National Party for using a bad copy of music from the rap hit Lose Yourself should be higher than the $600,000 the High Court ordered, the American copyright owner says.

Arguments in the Court of Appeal in Wellington on Tuesday centred around the damages the party and its national secretary Greg Hamilton were ordered to pay.

The party appealed against Justice Helen Cull's decision to award $600,000 damages for using a track called Eminem Esque, a "sound alike" version of the music to Lose Yourself, that breached the copyright.

NATIONAL The 2014 National Party advert showed a rowing team, with a voice speaking over an Eminem-style music track. (File photo)

But the submissions presented in public at the Court of Appeal avoided the dollar figures about music licensing deals that were given confidentially in the High Court before Justice Cull.

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The Court of Appeal reserved its decision.

ROSS GIBLIN/STUFF The National Party's lawyer, Greg Arthur, said the judge seemed to make no allowance for anything that was favourable to the National Party when she decided damages.

Two American companies, Eight Mile Style and Martin Affiliated, respectively own half the copyright, and administer it for Lose Yourself, performed by Eminem. They cross appealed, asking for the damages to be increased.

The National Party paid $4802 for the licence to use the music library track Eminem Esque, after a decision was made to look for music with a syncopated beat like Lose Yourself, to match the strokes of rowers in the campaign advertisement.

The music was used in a campaign advertisement for the 2014 general election. The similarity to Lose Yourself brought threats of legal action, and the music was replaced.

GETTY IMAGES Eminem was one of three creators of the original composition, Lose Yourself, but was not directly involved in the court case. (File photo)

The lawyer for the American parties, Garry Williams, said the amount of damages had to be proper and fair.

The political use was an element for which the National Party should pay more, he said. The owners had been asked to allow Lose Yourself for a different political ad, and permission was refused.

If the copyright owner would have refused to grant a licence for use, for instance for political purposes or for failing to give creative control to the copyright owner, that would be grounds for awarding additional damages, Williams said.

ROSS GIBLIN/STUFF The lawyer on the Lose Yourself side of the case, Garry Williams, said the damages should have been higher because the music was used in a political advertisement.

The National Party's lawyer, Greg Arthur, said the damages were based on the fee the party would have paid to use the music, for the 11 days it used Eminem Esque and making it available on the internet.

The party was not desperate to get the music so its offer would have been at the bottom end of the range, he said.

While it was keen to use the music when it cost $5000, it did not mean it would be also keen to use it at the figures now being talked about, Arthur said.

MAARTEN HOLL/STUFF One of the composers of the Oscar-winning Eminem song Lose Yourself, Jeff Bass, gave evidence at the High Court in Wellington where he played the guitar riff created for the song. (File photo)

The copyright holders have said they did not want to license the music for political use, and thought the party's ad "bland".

Arthur said Justice Cull seemed to make no allowance for anything that was in the party's favour in setting the damages.

The party was under the misconception, having taken advice from those who dealt daily with such issues, that having paid a licence fee to use Eminem Esque it was protected. The music industry had been "a little bit stunned" by the result, he said.

The judge increased damages for the political use but the evidence did not say there was a premium for political use, he said.

She had decided that Lose Yourself was a "highly original" work and the "soundalike" version Eminem Esque substantially copied it.

Her calculation of damages was based on using Eminem Esque, not the original of Lose Yourself, and included the extent of copying and and its relationship with the copyrighted music.

The National Party has said it will try to pass on any liability for damages to those involved in choosing, sourcing, providing, or licensing the Eminem Esque music.

Eminem Esque was the work of Los Angeles composer Michael Cohen, who was named as a party to the proceedings, but declined to become involved.