The National Party breached copyright by using a piece of production music in a 2014 election advertisement that resembled Eminem's 'Lose Yourself', a court has been told.

The hearing between Eight Mile Style and the National Party began in the Wellington High Court on Monday, and is set down for six days.

Eight Mile Style alleges the National Party breached several sections of the Copyright Act by using a piece of production music called 'Eminem Esque' it bought from a company called BeatBox.

The current hearing is set down to decide whether copyright was breached or not. Any decision on damages will be dealt with at a later date.

National has rejected the allegation.

In 2014, National claimed the track had been used on several television shows, and Eminem's publishers did not complain about any other use of it.

Steven Joyce also called the use of the track "pretty legal" - a statement which was famously mocked by comedian John Oliver.

Eight Mile Style LLC is a copyright administrator, which protects and enforces composers' rights as well as collects income on their behalf.

'Eminem Esque' was used in a National Party television advertisement featuring rowers during the 2014 general election campaign.

In his opening address, Eight Mile Style lawyer Gary Williams said the case was about the "wrongful use of an iconic piece of music".

"'Lose Yourself' is undoubtedly the jewel in the crown" of Eminem's catalogue, Mr Williams said.

The track won two Grammy Awards as well as the 2003 Academy Award for Best Original Song.

'Lose Yourself' was played in court as at least nine lawyers listened in silence. The track was then played again with no lyrics. Mr Williams said this was because the lyrics are irrelevant in a copyright case such as this - all that matters is the music.