The rapper says the song used in a 2014 ad by the governing party was unlicensed, while the party argues the music used was a track called Eminem-esque from a stock music library.
The case began on Monday with the tracks being played and listened to in court.
“'Lose Yourself' is a jewel in the crown of Eminem's catalogue," said Gary Williams, a lawyer for Eight Mile Style – a publishing group representing the artist.
Williams argued that the “iconic musical composition” had been wrongfully used, and has only been licensed for ads on a few occasions.
"Its commercial exploitation is tightly managed to protect the integrity of the work," he added. "It's known in the advertising industry that 'Lose Yourself' is not commonly available."
Songs that sound similar to famous tracks but different enough to avoid copyright infringement, are often found in free-to-use commercial music libraries.
However, Williams claims the use of the song had been a breach of copyright, and telling the court that emails from the National Party campaign team showed copyright concerns at the time, but ultimately decide the composer would be liable instead of them.
The musical composition and not the sound recording copyright is in question, according to Williams.
The National Party denies the copyright infringement as defence lawyer Greg Arthur said copyright was “not in any way proven by the name given to a piece of music."
The case is expected to continue for six days.