Image copyright AFP/Getty Image caption Jacinda Ardern has reportedly expressed a preference for Metallica's album Load

It's not every day you get tickets to see your favourite band from the prime minister.

But that's exactly what happened to William Bush, 12, who last year started a petition to bring the heavy metal group Metallica to New Zealand.

Six months and nearly 7,000 signatures later, the band announced a show in Auckland.

And on Friday New Zealand's PM Jacinda Ardern surprised the superfan with two free tickets to the gig.

"I am reliably told that your petition actually made a real difference to Metallica and the promoters," Ms Ardern told William when they met at Auckland's Mt Smart Stadium, according to reports.

She also thanked him on behalf of all the band's fans in New Zealand.

William's petition on Change.org, launched last September, said that "Kiwis just want a Metallica concert".

"It's been 8 years since you've visited New Zealand shores and we are all hoping for your return!" he wrote, saying another concert was "long overdue".

Six months later, Metallica has announced a show in Auckland on 31 October, supported by the heavy metal band Slipknot.

William heard the news on the radio but had no idea when he was invited to fly to Auckland that he would be meeting the Prime Minister.

"We may have to buy two more tickets," the Hawke's Bay resident told news site Stuff, saying it would be hard to choose between bringing his dad or his 15-year-old brother Ben.

Ms Ardern has expressed a preference for the heavy metal group, telling television network 1 News about her "pretty diverse tastes" and reportedly saying at her meeting with William that she hoped she could make the gig in October.

The world's youngest female leader has said in the past that she has a ukelele collection, and has snapped photos of the New Zealand Music Awards for her Instagram.

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