The five men returned to the service station, where the tourists were still waiting for a taxi, and set upon them, knocking one man to the ground. They concentrated their attack on him punching, kicking and hitting him with a window-washing brush. Eventually, the tourists broke free and were running up the road chased by Amtman's gang, when police arrived.
Amtman told police he retaliated after being attacked first.
Judge Paul Whitehead said Amtman had been the prime mover in the attack.
"It appals me that you would perpetrate such violence, and what's worse is that you went and got your mates to go back and do them over," the judge said.
"They would have gone home and told the people what a rotten place the West Coast of the South Island was. That can only harm every resident of the West Coast."
One of Amtman's associates, Ronald Bell, 19, had gone back to the service station to take part in the attack. He threw a number of punches and kicked the man on the ground.
Bell was a first offender but police refused to grant him diversion because of the seriousness of the incident. He was sentenced to 100 hours of community work.