The National Party has launched a new "fighting fund" now it is in Opposition - calling on supporters to donate and help stop a "political experiment" from taking the country backwards.

A party spokesman said an email sent out today calling for donations was part of the usual fundraising that all parties do, and not linked to the costs of the party being found in breach of copyright after its use of a song similar to an Eminem track.

In the email, campaign chairman Steven Joyce says National's 56 MPs will make up the largest Opposition in New Zealand's history.

"We will be working hard to hold the new Government to account, and ensure they don't squander the progress that New Zealanders have worked so hard to achieve."

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While details weren't available on many of the new Government's policies and associated costs, Joyce said it was clear they would take New Zealand backwards.

"The new Government is already bloated, with an executive of 31 Ministers and Undersecretaries. Hard-working New Zealanders can't afford this coalition Government."

Supporters are asked to chip in whatever they can to help National "as we begin taking on this Labour-Greens-NZ First 'political experiment'."

The National Party was found guilty last week of breaching copyright over its use of the track Eminem Esque in a 2014 election advertisement.

The track, which was created by a production house, was deemed to be "sufficiently similar" to Eminem's song Lose Yourself.

The High Court awarded damages plus interest from June 28, 2014, to 8 Mile Style, the publisher which controls some of Eminem's early catalogue.