The teenager lives with his father, who was not home at the time. Senior Sergeant Andy King said someone called 111 for medical assistance shortly after the incident, and ambulance officers then alerted police at about 5.20pm.
"The young man was injured quite badly. He was mixing a number of chemicals and items together to make a device and he was burned quite severely," Mr King said.
The teenager was taken to Middlemore Hospital in a serious condition . He remained in the burns unit for treatment.
Mr King said the outcome could have been fatal.
"I'd just really like to reiterate that this is a really serious incident resulting in really serious injuries," he said.
"Police, the Fire Service and medical staff are very keen for people to have fun with fireworks but to take a great deal of care with them. Also, avoid manufacturing and modifying them because that could result in very serious injury."
Anyone purchasing fireworks must be 18 or older.
They go on sale today and are only available until November 5.
Until 2007, the purchase age was 14 and sales were permitted for 10 days.
However, to enhance public safety, the rules were tightened.
Since then, the Fire Service said there has been a "dramatic reduction" in fireworks-related 111 calls.
"In recent years the only noticeable increase in fire-related calls has been on the nights that the event is widely celebrated - usually the two Saturdays closest to 5 November," said Assistant National Commander Rob Saunders.
Mr Saunders believes that a growing number of people chose to attend large, public displays rather than buy their own fireworks. "
"For those who do prefer to buy their own, the usual warnings apply. Use a torch to read and follow the instructions on fireworks before using them, never attempt to relight a firework that hasn't gone off, and always keep a bucket of water or a hose handy."
He also advised people to buy and use the small stands that hold fireworks safely, ready to be lit.
"Fireworks are noisy so consider neighbours and pets when letting them off, and never point fireworks at people.
"There are also some parts of New Zealand where open fires are restricted, so please check with your local council first before lighting any bonfires."
Between July 2008 and June 2011 $1,023,351 was paid to people who lodged ACC claims for fireworks-related injuries.
Burns made up 69 per cent of the injuries in the 2008/09 financial year, 72 per cent in 2009/10 and fell to 64 per cent in the year ending June 2011.
