The mother of a boy who died from carbon monoxide poisoning after a gasfitter signed off a hot water unit that shouldn't have been installed says the tragedy wasn't an accident and was preventable.

Justine Walker says losing her 12-year-old oldest son has left her permanently paralysed with grief and bitterness.

"It has ripped me apart. I am numb," she told the Herald, ahead of today's court sentencing.

"I feel there is only half of me left, how is this fair on my kids to have half a mum? I've lost everything. I feel helpless, scared unable to find peace or true happiness.

"I still wish every day for Jesse to come home."

Jesse Samuel died in a shower at his family's holiday home in Haast on October 10, 2018.

On the day of the tragedy, Jesse took a shower in the outdoor bathroom, opposite the kitchen where his mother was cooking dinner.

The second bathroom, which had an extraction fan, was used by family and extra guests. Jesse chose that shower because the water pressure was stronger than the one inside.

"My brother who gave us the shower was assured by the plumber and gasfitter it was fine for the heater to be installed inside. Now my brother feels bad for what happened. He blames himself," Walker said.

An autopsy found Jesse's blood level was 59 per cent saturated with carbon monoxide.

Gasfitter Michael Cartwright and Central Plumbing Ltd, which trades as Laser Plumbing Alexandra, are due to be sentenced today at the Dunedin District Court.

Walker is annoyed Cartwright has been able to continue operating his business.

Whenever she sees a Laser plumbing van driving by with the slogan "totally dependable" she feels "sick".

"I think, 'no you're not'. It is wrong he has a gas certificate and is allowed to keep on working. You teach your kids to own their mistakes and yet he can carry on - where is the accountability?"

Last year, Cartwright and his company pleaded guilty to two charges under section 125 of the Plumbers, Gasfitters and Drainlayers Act (2006).

The summary of facts said Gary Hale, a registered plumber and gasfitter who jointly owns Laser Plumbing Alexandra with Cartwright, was employed to install a gas hot water unit in a portable bathroom.

Cartwright, also a certified gasfitter, was responsible for signing off Hale's work.

Walker said there was no excuse for Cartwright not to check the unit before it was signed off.

"You have destroyed our lives, there is no peace and no end to the grief. This wasn't an accident. He shouldn't have signed off the job without sighting it. As far as I am concerned, he killed my son," Walker said.

Jesse's uncle Brendon Walker had bought the Bosch gas hot water unit in 2016, and was told it was suitable for the small bathroom unit. The bathroom unit was initially used at Walker's home in Alexandra while his property was being renovated.

The gas heater was described as an open flued, instantaneous water heater.

To avoid carbon monoxide building up, the unit had to be installed with adequate ventilation, the summary said.

Instructions for the appliance included a warning about how it should be installed. A warning sticker attached to the unit urged the installer to read the instructions, and stated the unit should not be installed in a bathroom.

Hale installed the water heater inside the bathroom and didn't raise any concerns with Walker. He was a tradesman gasfitter, so Cartwright was required to certify the installation.

The summary of facts stated Cartwright never visited the bathroom, didn't inspect the water heater, carry out any tests or consult with Hale.

Walker says Cartwright has not apologised to her or to her family.

Today she will read out her victim impact statement in court.

"He needs to hear what has happened to us.

"There is nothing right about this. It should never have happened. We were assured by them it was safe. I struggle to call this an accident. An accident is when you have done everything right and it ended wrong.

"There are so many wrongs here and who can we trust now? This process has taken two years of our lives. It will not end for us not even today. We have a life sentence."

Cartwright couldn't be reached for comment.

'His smiley face'

Walker says Jesse's smile would light up a room. Popular with his friends and a good all-rounder, Jesse loved technology and photography.

"He was energetic and loved to have fun. He was driven and excited about everything he did and wanted to do. I miss his smiley face, chats, and passion for life," Walker said.

On December 19, Jesse would have turned 15.

Walker, her partner Chris, and their children Cody, 11, Zane, 10 and McKenzie, 4, gathered at his grave to eat pizza and drink beers.

"Jesse would have been a typical teenager who would've loved that rite of passage.

"I think about Jesse every day and what he would have been like. But as the years go by it gets harder to imagine.

"Cody is 12 this year, the same age when Jesse died. It's not fair for him either. I think he feels Jesse was robbed of his life."