Eight Mile Style and LLC, who publish American rapper Eminem's music - sued the National Party over its 2014 election campaign ads which used the song Lose Yourself as a backing track.
The trial begins at the Wellington High Court this morning.
The National Party used the track without permission, and the publishers have taken them to court for Copyright infringement.
It was a highly unusual day in the High Court; Eminem's Lose Yourself plays to a room of unsmiling lawyers and a High Court Justice.
While rap is not usually played in court, this song is the centre of a legal battle between the National Party, and those who own the copyright on the song.
A backing track used in a 2014 National Party campaign ad was alleged to be an unlicensed version of Eminem's song, and today the case is being heard at the High Court in Wellington.
Lawyer for the plaintiff Eight Mile Style, Gary Williams, said the National Party infringed copyright by using the song, or a substantial reproduction of it.
He said that even if the party only authorised the infringement, that in itself would be a breach of copyright.
Mr Williams said it did not matter if the song's lyrics were not used.
He said there were three layers of copyright, covering lyrics, musical composition, and then the sound recording itself.
"The song Lose Yourself, is without doubt the jewel in the crown of Eminem's musical work," Mr Williams said, in his opening statements to the court.
"Not only did the song win an Academy Award for best original song in a movie, it also won two Grammy awards.
"In short, Lose Yourself is an extremely valuable song.
"The licensing of the song has been extremely carefully controlled. Despite many requests, it has only rarely been licensed for advertising purposes.
"When licensed, it can command in the millions of dollars. That's how valuable it is."
Mr Williams said there was evidence that the track the National Party used had previously been named Eminem ABBR, which they took as meaning "Eminem abbreviated".
He said that track name was later changed to "Eminem-esque".
The full track used in the National Party advertisement was also played in court.
The advertisement itself was then played.
The case is set down for six days.
