Six people have been injured – some of them critically – when a gas explosion destroyed a house in the New Zealand city of Christchurch, sending debris flying 100 metres in all directions.

The house, in the suburb of Norwood, was destroyed – described by one witness as “matchsticks … totally blown apart”.

The blast, which happened about 10.20am on Friday, was heard across the South Island city.

Witness James Looyer told New Zealand news website Stuff he was working nearby when he “heard the biggest bang we’ve ever heard”.

“We shot around there, peeked over the fence and there was the bodies of the people just hanging out of the woodwork and all the framing of the house. We just hopped the fence, and pulled all the wood off and dragged them out of there pretty much,” he said.

“We were trying to talk to them and they were just stone cold … they can’t even hear anything we were saying, just looking straight at us.

Christchurch remains on edge four months after a lone gunman killed 51 people and wounded dozens in attacks on two mosques, in New Zealand’s worst peacetime mass shooting.

There was no indication that Friday’s blast had any wider security implications.

Suspicions for the cause of the explosion have focused on the house’s gas heater. Devastated relatives of the residents said it had been causing problems and they were waiting for it to be repaired.

Police closed roads and evacuated surrounding houses after the blast. Evacuated residents were taken to a nearby country club.

Club staff member Tom White said concrete tiles from the roof of the house had been blown 100 metres in every direction.

“The house is matchsticks. It’s totally blown apart,” he said.

“It’s caused a lot of destruction to other houses – it’s blown the garage door right off the next-door neighbour’s house.”

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A resident of a nearby street told The New Zealand Herald that the explosion shook his house and knocked items off his windowsill.

“We’re OK but there are houses closer which are stuffed,” he said.

“The house where the explosion was, you wouldn’t even recognise it as a house now.”

St John Ambulance territory manager Craig Dowling confirmed to the newspaper that six people had been taken to hospital as a result of the explosion.

Their injuries ranged from serious to critical, he said.

A Christchurch hotel worker, who was not identified, said a sound like an explosion had shaken nearby buildings.

“It was more than an earthquake – you’d think a bomb had gone off,” the worker said.

-with AAP