Lawyers for American rapper Eminem and the National Party were back in court today, over the copyright of music featured in the party's TV advertisements during its 2014 election campaign.

Eight Mile Style LLC and Martin Affiliated LLC, Detroit-based publishers of Eminem's copyrights, accused the party of using backing music to the rapper's song Lose Yourself, from his 8 Mile album.

Greg Arthur who appeared for the party, told Justice Brendan Brown in the High Court at Wellington he wanted to split trial into two parts - one for liability and one for damages.

Mr Arthur said the two issues were "clearly divided" and a split trial would save the time spent in trial.

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He said the liability trial would take about two weeks, possibly more, but calculating damages "wasn't straightforward" so liability should be established first. Garry Williams representing Eight Mile Style LLC and Martin Affiliated LLC, disagreed, saying his clients wanted a single trial.

He said the assessment of damages wasn't complex and it could be dealt with in a single trial.

"They are issues that arise in copyright cases all the time," he said.

Justice Brown held off his decision and gave the parties the opportunity to come up with a resolution by March 23.

He said this was an unusual case. The National Party had flatly rejected the allegation it used the title track from the Eight Mile film, with campaign manager Steven Joyce saying in September he thought the use of the song was "pretty legal".

"We think these guys are just having a crack and have a bit of an eye for the main chance because it's an election campaign," he told reporters at the time."

In September, spokesman for the publishers Joel Martin said Eminem was never approached for permission to use his work in National's rowing-themed election ads, which featured backing music similar to the riff of Lose Yourself.

"It is both disappointing and sadly ironic that the political party responsible for championing the rights of music publishers in New Zealand by the introduction of the three strikes copyright reforms should itself have so little regard for copyright," he said at the time."