Five rugby clubs refused to play a rival team featuring patched Head Hunter gang members after claiming their players had been threatened with violence.

The boycott led to Whangaruru Rugby Football Club in Northland going gameless for the final rounds of the season before being beaten in their semi final.

Coach Hepi Haika said his team's dreams of promotion to the Northland premier club grade had been ruined by the boycott, which he said amounted to 'discrimination'.

He said their treatment by the Northland Rugby Union had been a "kick in the guts". "They are supposed to be there for rugby, not for political reasons."

But Northland Rugby Union chief executive James Parkinson said players from other clubs had felt afraid to play against the team.

"It's a legacy of years of incidents," he said.

Whangaruru had been leading the second tier Northland competition when a rival team complained to the Northland Rugby Union that their players had been threatened during a game and afterwards in a carpark.

A meeting was held and five teams - Tomarata, Southern, Ruawai, Kaihu Valley and Mangakahia - refused to play them.

"These are community clubs and they didn't feel safe, and didn't want to expose their communities to gangs," Parkinson said.

While the club had been well behaved on the field – receiving just two yellow cards in the season – gang paraphernalia had been worn at post-match functions.

"There has been no breach of any code of ethics, or on the field. It's just the perception of intimidation."

Whangaruru went five weeks without a game leading up to the semi finals, where they were beaten by a team they had previously scored 88 points against - a number which, coincidently, is a symbol used by the Head Hunters.

Haika, who has coached club rugby since 1982 and is a former Whangaruru player, said his hopes of returning his team to the premier grade before retiring had been crushed.

"They said they were threatened but none of them ever came up to me to notify me of what went down. It all came to a letter they wrote to Northland rugby.

"They took it in their own hands to boycott my team because of that when we'd been winning the competition."

Haika said the gang didn't have any official involvement with the club, but the two Head Hunters paid for the use of a training ground.

Gang sponsorship of grassroots sports clubs was of great concern to police.

Detective Inspector Virginia Le Bas said police were "very mindful of the influence these types of people can exert in all parts of our community".

"There are vulnerable people and communities that might be stoked to have money and or support offered to them, and they might become unwittingly associated."

Police investigations and prosecutions have found the Headhunters were making significant amounts of money through drug dealing, she said.

"It's not illegal to sponsor or support a club, and many sports teams struggle to get funding. But one would assume most people in the community would not want to be associated with an organisation that is involved in illicit drug activities, which causes significant destructive social long-term harm in their own community."

Parkinson said a full enquiry was underway by the Northland Rugby Union into the boycott. "If people feel strongly enough, they should probably be going to the police rather than us."

Haika said he would fight all the way to see his team play next season, and hadn't ruled out a complaint to the Human Rights Commission.