The case related to a song titled, 'Lose Yourself, ' an Original track of Eminem allegedly used by the National Party calling its song, Eminem Esque and used in the 2014 General Election advertising campaign.
The music used in the National Party ad was called Eminem Esque, copying and exploiting the original award-winning music.
High Court judge Helen Cull said the campaign song sounded like a copy and was a copy.
Eminem's company said it is "incredible that the party went to such great lengths to avoid responsibility for using a weak rip-off of Lose Yourself".
" We wish that we see even more initial music in advertising and marketing consequently and that authors obtain appropriately recognized as well as compensated for their effort".
Martin said he hadn't yet discussed the ruling with Eminem, aka Marshall Mathers III, but was glad the rapper hadn't been needed to travel to New Zealand "to watch the paint dry in the court room".
"This decision is a warning to "soundalike" music producers and their clients everywhere", said Simpsons Solicitors director Adam Simpson, who acted for Eminem's music company.
However, no additional damages were awarded because the National Party's actions were taken after getting professional, commercial, and media advice. He said the party purchased the music in good faith from an Australia-based library that had bought it from a USA supplier. "And "Eminem Esque" was one track on that shelf". It was found by a New Zealand court to have "substantially copied" his 2002 hit Lose Yourself.
He said the party was considering its next steps and already had lodged a claim against the suppliers and licence holders of the Eminem Esque track.
"Changing a few notes here and there just doesn't cut it".
Joel Martin, who spoke on behalf of the publishers, said the rapper was not asked for permission to use the song.