A gas contractor who worked on a Christchurch house just one day before it exploded arrived at the chaotic scene yesterday and made himself known to police officers.

The gas worker spoke to first responders at the scene and voluntarily made himself known, suggesting that police or fire investigators might want to talk to him.

He was devastated by what happened, sources have told the Herald.

It's believed the residents of the Northwood home were having trouble with their gas fire.

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Six people were injured in the explosion, which caused a blast so big it registered as a magnitude 2 earthquake.

The blast at 10.14am yesterday destroyed the house and flung debris across the neighbourhood. Emergency services at the scene said it was lucky no one was killed.

This morning, one person was in a critical condition and three people were in a stable condition in Christchurch Hospital. Another patient had been taken to Middlemore Hospital, where there is a serious burns unit, and another had been discharged.

Superintendent Lane Todd, the Canterbury Metro Area Commander, said police remained at the scene today. Police, the fire service, and Worksafe would be examining the scene today and tomorrow.

Todd said 12 properties were still unsafe and residents could not return home. Cordons remained in place.

Yesterday, frantic neighbours and passersby helped drag the survivors out of the debris.

They reported the victims had burns, cuts, scratches and bruises and were in a total state of shock.

Neighbour Bradley Culver was having a cup of coffee and watching the Open Championship golf on TV with his wife when the explosion happened.

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The former police officer ran outside and saw a neighbour's house destroyed.

He came to the trapped survivors' rescue.

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

"I am surprised nobody died [instantly]," Culver said.

"As a former police officer I've seen a lot in my days, but 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.

The property where the explosion happened is flattened and fire officials say six other properties have suffered major damage.

"The house is matchsticks. It's totally blown apart," Styx Mill Country Club worker Tom White said.

Fire and Emergency area commander Dave Stackhouse said all of Northwood was connected to piped gas, but at this stage it was not clear how there had been a leak.

"We've got our gas experts having a look at that," he said.

"Once we've got the scene safe, we'll look at what's caused this and carry out an investigation with police and WorkSafe."

Stackhouse said it was "probably lucky" the house inhabitants were close to the source of explosion when it happened.

"Often with these types of things it's the blast wave that can actually kill people further away from the actual radius of the initial explosion," he said.

"So I guess we are lucky that we are not looking at multiple fatalities here."