In a video posted on the festival's social media pages, organisers say "two different cultures met in a shared, emotional and powerful experience".
"More than 6000 metalheads yesterday, New Zealand Alien Weaponry welcomed Copenhell with the craziest haka greeting the band has ever experienced," the festival organisers wrote.
Harnett-Mutu also took to social media to thank the crowd for their amazing performance and said he was honoured to be given the opportunity to organise a 6000-strong haka.
"The crowd were so energised and incredibly respectful. They opened their arms and embraced their haka and through it... A deeper connection to Te Ao Māori and the band.
"An honour to tautoko your mahi, boys and to be part of this fantastic experience."
Its a wrap!.... Nga mihi nui to ALIEN WEAPONRY Henry, Louis and Ethan, Niel, Jette and the whanau. Sharing the kaupapa...
Posted by Kane Harnett on Friday, 21 June 2019
After the haka, drummer Henry de Jong told the camera: "I loved it, you guys did so well. The whole show, you guys were amazing.
"I don't think we've had a haka that size at one of our shows... to have a few-thousand people doing it is absolutely mind-blowing."
COPENHELL!!! ... What a AWESOME turnout from you crazy Danish MFs ... We LOVE your country ... And the haka you did for...
Posted by Alien Weaponry on Saturday, 22 June 2019
Alien Weaponry are well-known for their fusion of te reo Māori with thrash metal. The Northland group are currently on a world tour playing festivals such as Hellfest and MetalDays.