New Zealand's main conservative political party breached copyright by using a song similar to Eminem's Lose Yourself in its campaign ads, a judge has ruled.

High Court judge Helen Cull ordered the National Party to pay the Detroit rapper's publisher 600,000 New Zealand dollars (€351,460) plus interest.

The National Party ran a television ad that used the song "Eminem Esque" 186 times during its successful 2014 election 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 could have broader implications for other organisations that use so-called "sound-alike" tracks sold by commercial operators.

Ms Cull ruled the difference between the two songs was minimal and Eminem Esque reproduced the essence of Lose Yourself. She said it sounded like a copy and was a copy. She said it was no coincidence the composer of Eminem Esque had the music to Lose Yourself in front of him when he wrote his song.

She said Eminem's own lyrics in the song were ironic: "And prophetically so rapped Eminem: You better lose yourself in the music, the moment. You own it, you better never let it go."

Photo: JStone, Shutterstock

The judge based the amount of the award on a hypothetical licenc fee that the party might have paid to use the song. She noted that Eight Mile Style rarely grants permission to use Lose Yourself in advertising.

She stopped short of awarding additional damages, saying the party had only used the song after receiving professional advice that it could do so, and had not acted recklessly.

The National Party said in a statement it was disappointed with the ruling.

"We purchased the piece of production music from a reputable Australian-based music production library, who had purchased it from a US supplier," said National Party president Peter Goodfellow.

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.

The ruling comes at a difficult time for the conservative party, which just lost the 2017 election to a coalition headed by the liberal Labour Party.