The National Party is battling a ruling over how much it must pay for breaching copyright in a 2014 election ad, which played a song similar to Eminem's Lose Yourself.

Lawyers for the party are in the Court of Appeal in Wellington this morning, arguing the total damages of about $600,000 ordered by the High Court last year were too high, and that the judge took a "licensor-centric" approach.

The then-Government fought its case in May last year, accused of knowingly trying to sidestep licensing fees by using the track Eminem Esque.

The High Court ruled the similarities between Lose Yourself and Eminem Esque were so strong and it breached copyright.

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It noted publisher Eight Mile Style had exclusive control of the song's licensing, and rarely granted permission for the song to be used in ads.

Justice Helen Cull said the $600,000 would be the "hypothetical licence fee" that would have reasonably been charged for permission to use Lose Yourself in National Party advertising, including interest from June 2014.

In court this morning, lawyer Greg Arthur said an expert the judge relied on when setting a figure for damages had no relevant New Zealand experience, and her base fee was a "significant percentage higher" than it should have been.

He said a number of factors should have been taken into account when the figure was set, including the fact the National Party had alternative pieces of music it could have used instead of Eminem Esque.

The object for the party was to find a "syncopated beat" that matched the row strokes of the rowers in the video, not to copy Lose Yourself.

Given the music would only be used in New Zealand for 11 days, the party would not have gone to the top of the price range for music.

He said the party would not be "so desperate" for that piece of music that they would pay "above the odds for it".

Justice Cull "seemed to make no allowance for anything that was favourable to the National Party in this negotiation".

"It was entirely, I call it, licensor-centric.

"One gets the impression that her honour rather felt that Eight Mile Style was deserving of a significant remedy."

Arthur said Eight Mile Style could have sued for defamation or under the Fair Trading Act, but did not.

He said the judge considered the number of times Eminem Esque was used over the 11 days to be "intensive", but he estimated it to be about three times per day on each channel the ads were running on.

"[I] don't have any evidence on whether that's intensive."

There was also an issue around the territory a licence was used in - the judge had said territory did not matter because the ad ran on the internet, but Arthur said this decision was not supported by evidence.

"The National Party doesn't gain any benefit from internet use," he said, pointing out that the ad was targeting voting Kiwis, not people from other countries living overseas.

The High Court did not order additional damages because the National Party's actions were taken after getting professional, commercial and media advice. The court found the party had not been reckless.

The National Party bought Eminem Esque from a company called Beatbox, which in turn bought the licence from California-based music library Labrador.