christchurch gas explosion
Five days after a giant gas explosion reduced a Christchurch home to ruins, experts outside an official investigation are still at a loss to understand how it happened.

But an industry group representing gasfitters has told the Herald it will soon be requiring members to have in place a minimum liability of $2 million before they carry out work.

Nine homes were damaged when a sudden blast obliterated a Marble Court house in the northern suburb of Northwood, sending chunks of roof tiles and other debris over the neighbourhood.

Neighbours and others who raced to the aid of the six people inside at the time were amazed that no one was killed. As of yesterday, one man remained in a critical condition at Christchurch, while another person was in a stable condition at Middlemore Hospital, which had a serious burns unit.

A gas contractor who worked on the house, which was having gas issues, just one day before it exploded, arrived at the chaotic scene on Friday and fronted to police officers.

WorkSafe, which has taken over from police as the lead agency, was working to determine "the immediate and underlying causes" of the event - and the inquiry could take up to a year.

The organisation wasn't giving any initial indication as to what might have caused the blast.

What causes gas to explode?

David Williams, a professor in chemical sciences at the University of Auckland, said the explosive risk of leaked gas came down to the mixture of air and fuel.

When vaporised gas mixed with air, it came into contact with oxygen molecules, which, when ignited, created a fast reaction.

But what mattered was the critical lower explosive limit, which Williams said was generally around two per cent of gas to air.

"When the concentration is above that limit, then the gas can explode that - but if it's greater than the upper explosive limit, a flame won't propagate because there's not enough oxygen."

As it turned out, even a relatively small amount leaked gas could ignite an explosion.

"Inside a house, you'd only need a small fraction of the content of a normal gas bottle to cross the explosive limit," Williams said.

"Then all it takes is some sort of ignition, like an electrical spark from turning on a light switch or a thermostat."

In the event of a gas explosion, the blast had to expand rapidly and enormously - typically travelling toward the path of least resistance.

"If it happens outside, you hear a shockwave that sounds like a bang - but if you're inside a house, it's got to go somewhere, so it will take off the roof or a weak part of the house where it's contained, like a window," he said.

"These types of explosions are pretty phenomenal, as is obvious from the pictures in Christchurch. I experienced one myself once, when I was a building across the street from a tenement.

"The force of the explosion actually blew the windows into the building, while the windows on the other side of the building were blown out."

He expected the occupants of the Marble Court house - now little more than a heap of rubble - had been saved by being behind solid walls.

But he still questioned how the gas - which would have smelt of the odorant ethyl mercaptan added to it - wasn't noticed.

"I also wonder why it was leaking anyway, because appliances today have shut-off systems that are designed to prevent this kind of thing happening."

No cause for alarm - industry

They're questions that Master Plumbers, Gasfitters & Drainlayers chief executive Greg Wallace had pondered himself.

"The number one safety feature that LPG and natural gas have is that smell added to it, and it's ugly tasting smell - if consumers ever smell it, they should always know to turn the gas off and not attempt to light any appliance," Wallace said.

"But we are not involved in the investigation, so we are not going to jump to any conclusion and just let the investigators do the work."

Wallace said such events were extremely rare, and he couldn't recall one as catastrophic during his time in the industry.

Plumbers, Gasfitters and Drainlayers Board chief executive Martin Sawyers agreed such events were unusual.

"Gas explosions are relatively rare but they do occur and we are aware of other incidents," he said.

"A full house being demolished is very rare and we are only aware of one other similar incident where that has occurred."

Sawyers said there weren't any common factors across cases, and each had their own.

"At this time it is unclear what, or who, is responsible for what has occurred in Christchurch."

WorkSafe data included just one notified explosion, implosion or fire related to gas supply since 2016.

A handful of cases related to "escape, spillage and leakage of substance", although these figures were general and didn't contain other information, such as whether they occurred in homes or caused injury.

Some recent gas-related cases included a gas fire that severely damaged a Tauranga home after being incorrectly installed by local gasfitter Patrick Dykes, who in May was sentenced to four months' community detention and ordered to pay $20,000 reparation.

Four years earlier, Northland landlord Peter John McLeod was sentenced to six months' home detention and ordered to pay $5000 over the illegal DIY installation of a gas stove that leaked and caused an explosion which killed 19-year-old tenant Lesley Wehi-Jack.

Still, Wallace said when set against the sheer volume of gas customers in New Zealand - nearly 280,000 homes and businesses as at 2017 - the Christchurch blast was "a rarity".