The grieving mother of a 12-year-old boy who died from carbon monoxide poisoning in a gas-powered shower has spoken of her sorrow and warned other families not to use the shower units.

Justine Walker says she lost her "best friend" when oldest son, Jesse Samuel, died at their Haast holiday home three weeks ago.

"He was the most amazing boy. In life there are some special kids and he was one of them. He was kind, caring and helpful. He helped me through some tough times and he was always right beside me".

Jesse arrived at Haast with his grandmother a day after his mother and siblings on October 9. He was hoping to go whitebaiting with family, a hobby he loved.

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On the day of the tragedy Jesse decided to take a shower before dinner in the outdoors bathroom, opposite the kitchen, where his mother was preparing dinner.

The "second" bathroom which has an extraction fan, window, toilet and shower was used by extra family and guests. Jesse chose to take a shower there because the pressure was stronger than the one inside the crib. The shower was a gift from Walker's brother and there were no clues it was faulty.

"It was installed correctly by a plumber. My family would never put anyone at risk. A gas bottle is attached on the outside and a califont on the inside which heats the water. My brother had been using it at his holiday home for four months with no problem at all so there was no reason for something like this to go wrong at all."

Walker's two younger sons, Zane, 8, and Codi, 9, had also used it the day before.

"Jesse was in there for about 20 minutes but I didn't think too much of it because we were on holiday and I thought he was enjoying standing in the warmth. We have a big family so it's usually in and out of the shower at home. I wish I didn't let him use that shower now.

"It felt like it happened right before my very eyes. I know I couldn't prevent this but I feel so guilty. It's a mother's guilt that no-one can understand."

Jesse was overwhelmed by gas in the shower and lost consciousness. He was found unresponsive by his mother.

It is thought to be the first death of its kind in New Zealand and has sparked safety warnings.

Thousands of baches - and homes - around the country have similar gas continous flow shower water heating units. A spokeswoman from Master Plumbers said they were a "popular choice" for many homes and baches in New Zealand.

"They are easy to use and provide a reasonably cheap and reliable source of hot water," she said.

It was imperative they were fitted by trained professionals who were aware of "modern safety requirements".

Jesse would have turned 13 in December. The Year 8 student was looking forward to going to high school. Popular with his mates and a good all-rounder, Jesse was enthusiastic about technology and photography.

"He was a clever kid. Jesse and technology clicked," Walker said. "He knew how to fix the bluetooth in my car, the TV and our computers at home - he was talented in that way.

"He was an active kid. He'd always be on his mountain bike, playing cricket, rugby, ice hockey ... he tried everything. He would've been playing softball today. He liked a bit of gaming too but he was involved in so many sports I couldn't keep up".

Walker said snowboarding was Jesse's latest thrill.

"I was going to buy Jesse a snowboard for his [13th] birthday, but he got it a little bit earlier."

Jesse was farewelled by family and friends at the Alexandra Bowling Club on Wednesday. The snowboard was put on top of his casket so he could be buried with it.

"We played Jesse's favourite song Smoko by The Chats. He loved that song and would play it to his friends at school. The tributes were overflowing and Jesse was presented with three cups he would've received at the end of year prizegiving at Alexandra Primary School.

"But I keep thinking I should be standing at his 21st giving a speech, not at his funeral."

The Coroner's office could find no records of deaths in gas-powered showers in New Zealand.

But it said there had been two accidental carbon monoxide poisoning incidents - in 2011 which claimed the life of a man in his caravan at a camping ground in the Mackenzie Basin as a result from a leak in a poorly maintained refrigerator unit, and a Christchurch man who died in his home in 2010 after gas leaked from his heater.

Walker isn't sure she if she will ever go back to the Haast holiday home - the memories are too painful.

"I can't give up as much as I want to give up. I've still got three kids including my 2½ year old daughter, MacKenzie, who need their mum and their cuddles. I will miss Jesse, he was kind, and cheeky in a fun way with the best smile. I will miss him not getting married and having his own kids. He would have been an awesome dad. Everything he did he did well."

Master Plumbers said all gas appliances - including the shower set-ups - should also be serviced regularly "in line with the manufacturer's instructions by a licensed gasfitter to ensure that they are safe and compliant".