The cause behind a giant explosion that obliterated a Christchurch house in July last year has been revealed.

A gas fitter - halfway through repairing a gas fire - didn't disconnect its gas pipe from the supply and failed to tell the homeowners.

After he left and the mains were switched back on, 27 cubic metres of gas flowed into the property over 14 hours.

The next morning an electric thermostat on a hot water cylinder sparked, which sent a fireball tearing through the house.

The property was immediately destroyed, launching debris through the neighbourhood and injuring the six victims inside.

Gregory Smith came forward to police immediately and pleaded guilty.

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

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