The National Party has been found guilty of breaching copyright by using Eminem's track Lose Yourself for a 2014 election ad and must pay $600,000.
The High Court has awarded Eight Mile Style, the publisher of the hit song, damages, plus interest from June 28, 2014.
The High Court ruled that would be the "hypothetical licence fee" that would have reasonably been charged for permission to use Lose Yourself in National Party advertising.
The court noted Eight Mile Style had exclusive control of the song's licensing, and rarely granted permission for the song to be used in ads.
However, no additional damages were awarded 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 then-Government fought its case in the High Court in May this year, accused of knowingly trying to sidestep licensing fees by using the track Eminem Esque.
Now in a just-released decision, the High Court has ruled the similarities between Lose Yourself and Eminem Esque were so strong, it breached copyright.
The National Party says it is disappointed with today's verdict - and is now pursuing legal action against the supplier and licensor of the music.
The National Party bought the track Eminem Esque from a company called Beatbox, which in turn bought the licence from California-based music library Labrador.
National has now said it is considering legal action against the suppliers and licensors, while Eight Mile Style has a case for third party liability which was held back until there was a decision on its case against the National Party.
The lawyers acting for Eminem's music company say the decision "is a warning to 'sound alike' music producers and their clients everywhere".
Although the National Party legal team had tried to argue Lose Yourself wasn't a very original work to begin with, Justice Helen Cull found Eminem's Lose Yourself was "a highly original work".
"The distinctive sound of Lose Yourself is not limited by a 'melodic' line, but is a combination of the other instruments, particularly the guitar riff, the timbre, the strong hypnotic rhythm and the recurring violin instrumentation and the piano figure.
"It is no coincidence that Lose Yourself received the 2003 Academy Award for Best Original Song."
Cull ruled that Eminem Esque "substantially" copied Lose Yourself, to the point where the differences between the two were "minimal".
She particularly noted close similarities between the drum beat, the melody, and the piano.
Cull said that meant the National Party committed three breaches of copyright, by communicating a copy of Lose Yourself to the public without a licence, by authorising the copying, and authorising the use of the advertisements.
The two-week hearing in May that led to this decision often threatened to descend into farce, with lawyers grappling to find the line between imitation and copying.
Rap track Lose Yourself was repeatedly played to the courtroom at full volume.
Another musical interlude came in 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 was a serious debate over the complex workings of copyright law.
There was also the issue of setting a precedent on the legality of copycat tracks.
Eminem Esque was purchased from a licenced vendor, with defence lawyers pointing out that there were many such 'Esque' tracks available in the library.
The National Party was disappointed with the ruling, party president Peter Goodfellow said.
"However, the High Court found that before using the track the party took extensive advice and sought assurances from industry professionals that the track could be used by the party.
"The judgment has also found that the party in using the track did not act flagrantly or in a manner which justifies the party being further punished."
Goodfellow said National "purchased the piece of production music from a reputable Australian-based music production library, who had purchased it from a US supplier".
"The music was licensed with one of New Zealand's main industry copyright bodies, the Australasian Mechanical Copyright Owners Society [AMCOS]. Being licensed and available for purchase, and having taken advice from our suppliers, the party believed the purchase was legal.
"The party is now considering the implications of the judgment and the next steps. We already have a claim against the suppliers and licensors of the track."
Adam Simpson, who acted for Eminem's music company Eight Mile Style and writers Jeff Bass and Luis Resto, said the ruling was a warning to music producers everywhere.
"The ruling clarifies and confirms the rights of artists and songwriters. It sets a major precedent in New Zealand and will be influential in Australia, the UK and elsewhere," he said.
"'Sound alike' tracks are often used in advertising to be reminiscent of well-known original music, while avoiding appropriate acknowledgement and payment for the use of the original piece. Getting too close to the original risks infringing copyright.
"Eminem Esque clearly stepped over the line. It copied the essential elements that made Lose Yourself a global hit. It was calculated and intentional.
"Changing a few notes here and there just doesn't cut it."