An electrician halfway through repairing a gas fire didn’t disconnect its gas pipe and failed to tell homeowners who were lucky to survive a giant explosion that obliterated their Christchurch house not to use the gas.

The Herald can reveal for the first time just how gas leaked overnight through the house and caused a massive explosion that sent destructive shockwaves through neighbouring properties.

Witnesses and fire bosses at the time were amazed nobody was killed.

Gregory John Smith, sole director of Christchurch-based plumbing and gasfitting business Gas Unlimited Ltd, who had been doing the work, was phoned soon after the July 2019 blast by the fireplace manufacturers who he’d been talking to about getting a new part.

“I mustn’t have … I can’t have … I didn’t cap the gas. It was the fire we were working on yesterday,” a shocked Smith said on the phone, according to court documents obtained by the Herald.

He went straight to the epicentre of the blast scene and made himself known to police officers.

Smith was later charged with failing to take action required by gas safety regulations.

He pleaded guilty at Christchurch District Court in September and will be sentenced next week. A judge earlier said it’s likely he will face financial consequences.

The people inside the property at the time of the explosion, who are currently subject to an interim name suppression order, suffered varying injuries.

For the first time, the Herald can report just how the explosion happened.

In 2019, the property owner engaged Gas Unlimited to repair a gas fireplace heater, which was not working properly. When the homeowner tried to start it, the fan would run but the fireplace would not ignite.

A family member, who was an electrician, tested the fireplace and believed the problem might be with the solenoid – a valve component that controls the flow of gas from the inlet pipe through the fireplace. But they did not do any work because it appeared to be a gas rather than an electrical issue.

On June 17, 2019 Smith visited the property and examined the fireplace, which was displaying an error code on the controller.

He partially dismantled the fireplace but did not disconnect it from the pipework supply or cause any pipe to be open-ended, says the summary of facts obtained by the Herald.

Smith then called Escea – the fireplace manufacturers – who advised he send the computer board to their Dunedin office for testing.

After removing the computer board, Smith put the fireplace back together and left. He did not turn the gas off at all during his visit.

On July 10, 2019, Smith returned with the computer board which had been tested but found it still showed the same error code.

The homeowner, who was there at the time, suggested to Smith that it might be the solenoid, and explained what their family member had said.

Smith phoned Escea again who talked him through some fault-finding tests, which resulted in him suspecting that two of the solenoid coils were faulty.

He returned on July 18, 2019 with replacement solenoid coils but found they were not the right parts.

Smith located the gas mains outside the house and turned the gas off at the isolating valve.

He then disconnected the solenoid and called Escea again to discuss the issue.

An Escea technician suggested that Smith return both the new and old solenoid coils, the transformer, computer board and remote back to Escea so they could be tested as a set.

After the 54-minute phonecall, he left the property.

But the summary of facts says that after removing the solenoid, Smith “did not take any steps to isolate the gas supply (other than turning it off at the mains)”.

And he did not cap the appliance gas pipe or disconnect the fireplace from the gas pipe that supplied the appliance, or disconnect the installation and cap the supply pipework at the mains.

“Mr Smith did not say anything to [the homeowner] about what he had done with the fireplace or what she could or could not do with the gas,” the summary of facts says.

“In particular, he did not tell her that she should not turn the gas mains back on.

“Further, he did not place any type of labelling on the gas meter or take any steps to lock the gas valve to ensure that no one else could turn the gas back on.”

That evening, the homeowner had some friends coming for dinner and to stay the night.

When the homeowner tried to turn the gas hob on, they discovered none of the burners worked.

They realised Smith had not turned the gas back on at the mains after turning it off that afternoon.

With one of the visitors, they went outside and turned the gas mains back on.

After turning the hob on, they immediately smelt gas but turned on the range hood and lit a candle on the dining table to remove the smell.

They stopped noticing the smell after less than a minute.

After dinner and then drinks in the lounge, some of them stayed up until 3.30am to watch a netball world cup match on TV.

It appears none of them noticed a smell of gas after the initial smell when the mains were first turned on. The mostly likely explanation, included in the summary of facts, is the concept of “odour fatigue” or “olfactory adaptation” where someone adapts to a smell and stops noticing it over time.

However, gas flowed out of the open appliance gas pipe through the fireplace into the property “steadily throughout the evening and the following morning”, the summary says.

A total of about 27cu m of gas flowed into the property over about 14 hours. Over the previous three-month period, the property had consumed about 1cu m of gas.

“The concentration of gas inside the house may have built up at a slow enough rate for the occupants to become desensitised to it throughout the evening without noticing it,” the summary says.

“As gas had been flowing into the property throughout the evening, by the following morning there was a large volume of LPG resting inside the house.”

At around 10.10am, one of the visitors arose and turned on the electric jug and toaster in the kitchen.

The action of walking through the house “disturbed the low-lying LPG”, causing it to mix with the air and “bringing it to within its explosive range”.

He returned to the bedroom.

The electric thermostat on a hot water cylinder in a linen cupboard near the centre of the house disengaged at 10.15am and “created a brief arc, generating a spark which ignited the gas/air mixture and caused it to explode”.

“The explosion created a large blast pressure wave followed by a large fireball which emanated from the property,” the summary of facts says.

“The property was immediately destroyed entirely. The shockwave caused damage to a number of neighbouring properties. Heavy debris from the explosion was thrown up to 100m from the property, and lighter debris was located throughout the neighbourhood.”

On the day of the explosion, St John said six injured people were rushed to hospital with various injuries.

When Smith was interviewed by WorkSafe New Zealand, he acknowledged that he had failed to isolate the appliance from the gas supply.

He expressed a willingness to apologise to the victims.

Smith and his company Gas Unlimited will be sentenced at Christchurch District Court next Tuesday.

At the time of the near-fatal tragedy, a neighbour told the Herald he was having a cup of coffee and watching the Open Championship golf on TV with his wife when the explosion happened.

He ran outside, saw a neighbour’s house completely destroyed and came to the trapped survivors’ rescue.

They were stunned, he said, hardly able to talk.

“I am surprised nobody died [instantly],” he said.

“… I’ve never seen anything like that.”

The blast sent roof tiles, glass, wood fragments and pink insulation material flying into the air and raining down across several streets.

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