Hamiora then pushed his T-shirt between the bars of the portable LPG gas heater when the garment caught fire.
Reihana, a solo dad, said his son knew his plastic toys melted on fire so thought his son expected his T-shirt would also melt.
Suddenly the family heard "a little moan" from the next room and his sisters went into the lounge to find Hamiora's shirt was on fire.
Reihana raced into the room and carried his son to the bath where he ran cold water over him. He then dialled 111.
Firefighters have praised Reihana's quick actions, but the Te Kuiti father-of-three said he was just acting on instinct.
"I've burnt my thumb and finger and put it straight under the cold water or in the fridge or on the ice."
Hamiora was flown to Waikato Hospital by the Westpac Waikato Air Ambulance on Friday night and later transferred to Middlemore Hospital's National Burn Service unit where he remains.
The youngster had only just started at Te Kuiti Primary School in May when he turned 5 and his father described Hamiora as a real character with attitude.
Reihana had been at his son's bedside since the accident and said the little boy was in a bad way. His two daughters were being cared for by an aunty in Te Kuiti. They were both missing and thinking about their brother.
"My aunty calls me up in the morning and then at night just to talk to them. They say, 'How is Hamiora' and I say, 'He's getting better'."
Reihana thought Hamiora could hear them because he moved about when anyone in the family spoke to him.
"I hope he gets better quick because I know he's a good fighter. He's a tough little fella, he's a tough little guy."
A family friend, who did not want to be named, described Hamiora as "a real character".
"He wears glasses off the end of his nose, he's quite daggy. He's very spontaneous. He had just started school ... he was very ready to go to school and really enjoying school.
"They'll (the family) all be pretty traumatised."
A resident in the street where Hamiora lives said she had seen the siblings playing often and described the family as nice people.
Waikato specialist fire investigator Kevin Holmes could not fault Reihana's actions.
"He did everything right. You smother the flames or roll them in a blanket and then put them in cold water for 20 minutes."
Holmes said because Hamiora had only started school this year he was probably too young to have seen the New Zealand Fire Service's Get Firewise programme which teaches primary school children about fire safety.
If there was anything to be learned from the tragic accident it was to "keep an eye on your kids around heaters".
Anything combustible, whether it was clothing, furniture or a person, needed to be at least one metre from any type of heater, including fires.
"It's a horrible accident. Just a terrible, terrible accident. You wouldn't wish it on anyone."
A givealittle page has been setup to help Hamiora and his family with his recovery. Visit https://givealittle.co.nz/cause/helpforhamiora
Heater safety tips:
• Remember the heater-metre rule -- always keep furniture, curtains, clothes and children at least one metre away from heaters and fireplaces
• Don't store objects on top of your heating appliance
• Never cover heating appliances
Portable LPG gas heaters
• Make sure the ceramic heater element is not broken or chipped and that the element guard is in place
• Check to see that the hose is in good condition and doesn't show any signs of damage or wear
• If the heater does not light straight away, turn it off and then try again. Don't let the gas build up before trying to relight it
• Always have fresh air coming into the room where a gas heater is in use
• Have your heater serviced every 12 months
Source: New Zealand Fire Service
