A pop music expert is warning people of using 'sound-alike' tracks sold by commercial operators.

It comes after the National Party was found guilty of ripping off Eminem's song 'Lose Yourself' in its 2014 election campaign ad.

It's been fined $600,000.

The High Court judge said the campaign song reproduced the essence of the rapper's hit, ruling there was minimal difference between them.

Dr Nick Braae, from Waikato Institute of Technology, said music creators need to be careful when using other peoples' work for inspiration.

"I think there's still a place for people to pay tribute to artists that influence them and to borrow things from other sources but it has to be done in more subtle ways, you know bring different ingredients together to create a new product."

Simpsons Solicitors director, Adam Simpson said it's a cautionary tale, that might serve as inspiration to invest in new music.

"I hope it means we see more original music in advertising and those people who do want to use sound-alike's do so much more cautiously."