A group of youths chanted "drown him" as they held the head of a 15-year-old under water.
The teenager was brutally beaten up in Rolleston, Canterbury region before being dragged to a water race and pushed under.
Police are following strong leads in identifying the attackers.
Said Senior Sergeant Pete Stills: "He has received a real pasting. His eyes were swollen shut and he needed stitches for a pretty serious gash underneath his chin," he said.
"It was a harrowing attack by a group on a lone individual, "completely unprovoked and cowardly".
The attack, which happened on Saturday night, appears to have been robbery-motivated.
Four youths approached the 15-year-old on Tennyson St, Rolleston, at about 10.30pm and demanded his cap, Senior Sergeant Stills said.
He had earlier been at a nearby fireworks event in Foster Park.
When he refused to hand over the cap, the group repeatedly punched him and kicked him on the ground.
They dragged him to a nearby water drain and held his head under the water.
Then in an appalling move, some in the group urged the others to drown him.
"We are unsure how long he was underwater for," said Senior Sergeant Stills.
"At one stage and he believed he was going to be drowned."
The teenager eventually broke free and attempted to run home.
The teenager's mother, who did not want to be named, said he collapsed near their house and caught the attention of the neighbour's who brought him home.
"I didn't recognise him, I thought 'who the hell are you'," she said.
"He couldn't talk and his mouth was full of blood."
They called 111 and he was taken to Christchurch Hospital.
The teenager was discharged from hospital on Sunday.
Anyone with information can call the police on 363 7400 or anonymously through Crimestoppers on 0800 555 111.
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