A legal battle between the National Party and musician Eminem's publishers has kicked off in Wellington with a court room full of lawyers being treated to some rap music.

Detroit-based record companies Eight Mile Style and Martin Affiliated are suing the party over music it used in an 2014 election ad.

The song in question has been compared to the American rapper's 2002 hit Lose Yourself and Eminem's representatives say permission was not sought nor given for the song to be used in connection with any political campaign.

As the case kicked off in the High Court at Wellington on Monday, the court was played the full version of Lose Yourself, with lyrics, followed by a track whose file was allegedly labelled "Eminem-esque", before the instrumental of Lose Yourself was heard.

"Lose Yourself is a jewel in the crown of Eminem's catalogue," the plaintiff's lawyer, Gary Williams, said.

He said the "iconic musical composition" had been wrongfully used - and despite many requests in the past, had only been licensed for ads on a few occasions.

"Its commercial exploitation is tightly managed to protect the integrity of the work," he said.

"It's known in the advertising industry that Lose Yourself is not commonly available."

He said the file for the song on the commercial may have originally been labelled

"Eminem-abbr", but later renamed "Eminem-esque".

It was not the sound recording copyright in question, but the musical composition, Mr Williams said.

National's lawyers say the song was part of a library bought from production music supplier Beatbox.

The trial is set down for six days.