Expert investigators are piecing together how a suspected gas leak caused a massive explosion in Christchurch today that injured seven people.

Witnesses and neighbours who felt the "shockwaves" of the giant blast in the northern suburb of Northwood's Marble Court around 10.20am are stunned nobody died in the accident.

The modern brick house where the explosion happened – and was felt across the city - has been totally destroyed.

Five people were inside the house which reportedly had been having gas fire issues.

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St John said six injured people were rushed to hospital. One person is in a critical condition, three are stable, one has been discharged, and another has been flown to Middlemore Hospital in Auckland where there is a serious burns unit.

Frantic neighbours and passersby helped drag the survivors out of the debris.

They reported the victims having burns, cuts, scratches, and bruises and being 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.

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

He came to the trapped survivors' rescue.

They were stunned, in total shock, he said, hardly able to talk.

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

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"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.

While the property where the explosion happened is completely flattened, 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.

Indraj Fonsaka was nearly blown off the toilet.

"It nearly knocked me off. I thought it was an explosion underground," said Fonsaka, who lives across the street from the blast.

His wife Akita was screaming from the garage.

They ran to each other and couldn't believe what they saw. They both thought it was a meteor strike.

Mary Messeha was boiling eggs for son Jacob's morning snack.

The 20-month-old was playing in the lounge while elder sister Lily, 9, chilled out on the iPad.

Just moments before the enormous explosion rocked their house, Jacob had been playing near a large wall clock.

"Suddenly there was a very big, loud noise and the clock came crashing down," said their mum as she recovered from the traumatic event at a friend's place.

"We were all freaked out, especially the kids."

After the explosion, which she thought must have been a giant earthquake, she ventured outside.

Neighbours were crying and dazed. Rubble – roof tiles, insulation, wood fragments – were scattered across the street.

Messeha could see flames and smoke coming from a house over on the next street, Marble Court. She asked a stunned Brockwater Ave neighbour if it had been an earthquake.

"They said, 'No, it was an explosion'. My first thought was that it was a terrorist explosion and I was so scared," she said.

Messeha is unsure when she will be allowed to return home. But for now, she's just grateful it wasn't even worse.

"I'm happy it was a gas explosion and not a terror attack but I feel so sorry for those people who own the house," she said.

"I hope they are okay and getting through this. It's just a terrible thing to have happened."

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."