The case, which began in May, is the latest to test the legality of so-called sound-alike music.
Back in 2014, the National Party aired a campaign ad highlighting the nation's flourishing economy with a background track titled "Eminem Esque" that it used to drive home their message.
As previously reported, New Zealand's conservative National Party was accused of ripping off "Lose Yourself" for a 2014 campaign ad.
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.
The judgment considered the drum patterns, background chords and violin tones of each version, all of which it said bore "close similarities".
Joel Martin, who spoke on behalf of the publishers, said the rapper was not asked for permission to use the song.
Lose Yourself is Eminem's signature song and biggest hit, having topped Billboard's Hot 100 for 12 weeks in 2002-2003 while also winning the Academy Award for best original song. The judgement has been handed down and the National Party now owe Eminem $NZ 600,000.
"Changing a few notes here and there just doesn't cut it". "They knew we would not have permitted the use of the song in their political advertisement; however, they proceeded at their own risk and blamed others for their infringement".
National Party President Peter Goodfellow said that his Party was disappointed with the High Court decision.
"The party is now considering the implications of the judgment and the next steps".
"This decision is a warning to "sound alike" music producers and their clients everywhere", said Adam Simpson, director of Simpsons Solicitors, who acted for Eminem's music company, Eight Mile Style and writers Jeff Bass and Luis Resto.
"We already have a claim against the suppliers and licensors of the track", he said.