A Hamilton high school student needed nine stitches after being struck in the back with a suspected skyrocket.
The youth was walking up a hill at Hamilton Lake when he heard a loud explosion and something hit his back, police said.
He said he was hit with askyrocket or some other type of firework.
Police said they were also called to a house in Claudelands, Hamilton East, after residents reported what they thought was an exploding bomb.
Two teenagers were found setting off fireworks in glass bottles in their backyard.
Police said they had attended more than 50 incidents involving fireworks this week. Senior Sergeant Paul Jermy said many of the incidents could have had serious consequences.
"In many cases they are just young guys being stupid and reckless and having total disregard for people's safety."
People could be charged with wilful damage, disorder, assault with a weapon and charges related to endangering safety, transport and property, he said.
Police in Christchurch have charged five youths with arson after two fireworks-related fires .
Police said a toilet at Crosby Park in suburban Avonhead was set alight on Thursday.
A detective working in the area saw smoke from the building and used a fire extinguisher to quell the blaze. Three youths were arrested nearby.
Two other youths were charged overnight with setting fire to a suburban Burnside building early last Saturday morning.
Detective Senior Sergeant Grant Wormald, of Papanui police, said fireworks had been used in both arsons with other combustible materials.
He warned parents to ensure they knew where their children were and pointed to penalties for arson of up to 14 years' jail.
"Given the potential for serious damage to property and the risk of injury or loss of life as a result of arson, the police investigate all such offences," he said.
"As demonstrated by the arrests made, the chances of being caught are high."
The arrests came after a spate of fireworks-related incidents around the country in the lead-up to Guy Fawkes day tomorrow.
