The boy suffered severe scalding to his entire body when he fell or jumped into a 212F (100C) pool in the North Island tourist resort of Rotorua.
Witnesses heard him screaming in agony and saw his skin peeling off during the Boxing Day incident.
He was airlifted by helicopter to the intensive care unit at Middlemore Hospital in Auckland.
The boy, who was of Pacific Island heritage, was with his family visiting Rotorua's Kuirau Park, a geothermal hot spot pockmarked by boiling pools of water and natural cauldrons of bubbling mud.
Fences surround the geothermal features and there are warning signs in the council-run public park.
A hospital spokesman said the boy's family were with him when he died.
Anna Kare, a Frenchwoman who runs a café in the city, said she was walking through the park with her mother-in-law and two daughters when she heard screaming.
She saw a teenager, whom she assumed to be the boy's brother, running through the park with the child on his back.
"At first I thought they were just playing until I heard what he was saying," Mrs Kare said.
"There was screaming and yelling. It was just horrible.
"The boy was yelling 'my hands, my hands', and then I saw that all the skin was peeled off his hands."
Garry Page, Rotorua District Council parks and recreation manager, said: "Apparently this boy had actually climbed up on to the fence and jumped into the pool.
"There's not a lot we can do to stop that if someone is that determined to get over the fence."
Mr Page said the incident was under investigation.
Rotorua, which lies at the heart of a geothermal plateau, attracts hundreds of thousands of tourists each year to its geysers and related natural wonders.