A groom was among a group of people who chased and bashed to death a Northland man after an argument in Kaitaia.

Mongrel Mob gang member Christopher Manuel married his partner in Kaitaia on New Year's Eve 2015. His wedding attire was a red Mongrel Mob vest over a T-shirt, a pair of red shorts, a black bum bag and a Nazi-style helmet.

He wore the outfit for much of the day. In the evening, he was part of a group that chased and seriously assaulted Lee Rata - who was also a Mongrel Mob member - after an argument on Bonnetts Rd, Kaitaia, about 120m from the wedding venue.

Yesterday, Manuel, 36, and Lester Pairama, 17, pleaded guilty in the High Court at Whangarei to manslaughter and aggravated robbery.

Both were initially charged with murder and their trial, with four others, was scheduled to start in court on Monday.

Juanita Rafael Wilcox, 26, and Joseph Martin, 41, admitted being accessories to murder.

Manuel and Martin also pleaded guilty to conspiring to pervert the course of justice.

The trial for two others - Gregory Mark Comer and Gregory Matthew Comer- will now begin on Monday. They are charged with being an accessory to murder and Gregory Mark Comer is also charged with attempting to pervert the course of justice.

The court heard that several patched and prospective Mongrel Mob gang members from Kaitaia, Moerewa and Auckland attended Manuel and his partner's wedding and stayed for a reception in the evening.

Rata left home wearing a leather vest with red strings down the side, a baseball cap and a dog chain wrapped around his torso, believed to be for self defence. He had a knife and also wore other Mongrel Mob gang-affiliated clothes.

He had been drinking that afternoon and evening. Rata was not invited to the wedding but spoke to several people near the party venue and had earlier smashed a glass on the ground after an argument with guests, including the bride.

A group, including Pairama and Manuel, chased Rata and yelled at him to pick up the smashed bottle.

Manuel then struck Rata with a closed fist two or three times before he was repeatedly kicked and stomped on about his head and body as he lay on the ground.

One of the attackers used what police believed was a Nazi-style metal helmet to strike Rata around his head.

Pairama then removed a dog chain and a leather vest as Rata lay unconscious.

The injured man was taken to Kaitaia Hospital and then to Whangarei Hospital where he died the next day after his life support was switched off.

The four men will be sentenced on December 1.