It's free to sit in on court proceedings, but a cover charge wouldn't have been out of place on Monday at the High Court in Wellington.

Hits from artists including Megadeth, One Direction, Slayer, Coldplay and Justin Bieber rang out as the legal rap battle between Eminem and the National Party raged on.

Dr Kirsten Zemke, University of Auckland ethnomusicologist appeared as an expert witness for BeatBox, the company which licensed the track 'Eminem Esque' to the National Party for its 2014 election campaign.