New Zealand's National Party has been ordered to pay Eminem more than $500,000, after a court ruled a song used in the party's 2014 election campaign was a breach of the US rapper's copyright.

Key points: National Party used the song 186 times before pulling the ad off the air

National Party used the song 186 times before pulling the ad off the air The judge found the two tunes were "strikingly similar"

The judge found the two tunes were "strikingly similar" Eminem entitled to damages of $535,000, with interest, from June 28, 2014

The track, used during the conservative National Party's successful 2014 election campaign and titled Eminem Esque, was found to have "substantially copied" Eminem's hit song, Lose Yourself.

The National Party used the song 186 times during the campaign before pulling the ad off the air.

Publisher Eight Mile Style sued, saying the track ripped off the rapper's acclaimed 2002 hit.

The case was filed in 2014, and the court heard the songs and arguments in May.

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"The close similarities and the indiscernible differences in drum beat, the 'melodic line' and the piano figures, make Eminem Esque strikingly similar to Lose Yourself," Justice Helen Cull wrote in her final judgement.

The court found Eight Mile Style was entitled to damages of $NZ 600,000 ($535,000), with interest, from June 28, 2014.

The court said the publisher had exclusive control over the song, and rarely granted permission for the song to be used in advertising.

However the party was not ordered to pay further damages because the court found it had not been reckless as it sought professional, commercial and media advice.

Beat drops and 'doodlings': a thorough analysis

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Justice Cull said the song, which won Eminem an Oscar for its role in the movie 8 Mile, had "a heightened irony in the context of these proceedings".

She said the refrain of "You better lose yourself in the music, the moment, you own it, you better never let it go" was rapped "prophetically" by Eminem.

The court's judgements were summarised in a 132-page ruling, where 'expert musicologists' delivered their verdicts on the similarities between the two tracks — often using slang.

"At 1 [minute] 38 [seconds] of Lose Yourself the beat 'drops' after tension and anticipation," witness Dr Kirsten Zemke said.

"Eminem Esque has no such drop."

The University of Auckland's Dr Zemke referred to the "doodlings" of the piano in both songs, saying they were most likely altered intentionally, "enough for the work to not copy anything that would qualify as 'melodic'".

Justice Cull also took her own musical taste into account.

"The ear tells you Eminem Esque sounds the same and the listener is left thinking one has come from the other," she said.

Eminem has achieved notoriety for his controversial lyrics and unapologetic style.

Earlier this month, he launched an expletive-laden attack on US President Donald Trump during a four-minute freestyle rap unveiled at the BET Hip Hop awards.

Sorry, this video has expired Eminem slams Trump in freestyle rap for hip hop awards show

The National Party's court loss came only days after an electoral defeat, as Prime Minister Bill English failed to form a coalition with so-called 'kingmaker' Winston Peters.

Mr Peters' NZ First Party, which held the balance of power after September's inconclusive vote, chose Jacinda Ardern's Labour Party to lead the country.

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