National bought the music for $4800 from Beatbox, a licensed music company from Australia. In court it argued it had done so in good faith and had sought assurances that there was no risk of copyright infringement.
But Eight Mile's lawyer Gary Williams said the National Party infringed copyright by using the song, or a substantial reproduction of it.
Williams said even if National only authorised the infringement, that in itself would be a breach of copyright.
The song was "without doubt the jewel in the crown of Eminem's musical work", winning an Academy Award and two Grammy awards, he said in his opening remarks in a court hearing earlier this year.
"The licensing of the song has been extremely carefully controlled. Despite many requests, it has only rarely been licensed for advertising purposes.
"When licensed, it can command in the millions of dollars. That's how valuable it is."
The two-week court case played out in May to world-wide publicity, with US comedian John Oliver poking fun at New Zealand and the National Party, which has been the butt of his jokes for some time.
On May 12 Justice Helen Cull reserved her decision, noting that such decisions usually took up to three months to deliver.
There has been no comment from the courts on why the decision has taken more than five months to be released, leading to speculation the delay was politically motivated to avoid the election period.
The judgement is due to be released at 3pm on Wednesday.
Watch: National party ad
Watch: Eminem's 'Lose Yourself'
