The man and woman in the small Auckland sleepout could smell gas when they woke at 4 this morning.
But they went back to sleep.
The smell got stronger as the invisible gas filled their home, which is tucked down the end of a drive behind the house at 59 Mary Dreaver St.
Just past 6am the man got up and flicked on a light.
Then the house exploded.
The blast ripped off all the walls and the corrugated iron roof, part of which ended up in the garden next door. Fire services said the house's back wall was hurled 30m by the force of the blast.
The boom shook neighbouring houses and roused residents in streets around New Windsor and Blockhouse Bay. Local James Wheeler said he was woken about 6am by an "almighty bang".
"It seemed like a thunderbolt had hit my house," he said.
Neighbour Jarrod Darlington lives barely 20m away; at first he thought a car had ploughed into his home.
He heard screaming and rushed out to see the sleepout partly levelled and flames racing through the ruins.
"You could see the house had split and it looked like the back of it had collapsed."
Neighbours rushed to the aid of the couple living there, he said.
Darshana Dabhoya lives across the road at number 56.
"We were just sleeping and I heard a big blast. I could hear a lady screaming really bad ... really scared. And I could also see the flames and smoke coming from the house."
Incredibly, both people escaped the fierce blaze. Avondale senior fire station officer David Wood said they were lucky to get out alive.
The 42-year-old woman had serious burn injuries, and the man had moderate injuries. Both were in Middlemore Hospital in a stable condition tonight.
Homes around the property were evacuated as a precaution, some residents leaving still dressed in their pyjamas.
It took six hours before fire investigators were able to enter the property, which Wood said had been "totalled".
In the charred, smouldering remains of the building, a kitchen bench with a sink and plates is still visible.
That's a major red flag - because the sleepout was not consented for a kitchen, meaning any gas installation would have been illegal.
Auckland Council's building control compliance manager Sally Grey said the former Auckland City Council had granted building consent for the sleepout in 2001.
"If any sanitary facilities or a kitchen were added to the sleepout after the original consent was signed off, it would have been without consent or council oversight," she said. "We will investigate further."
Fire investigators were working overnight to determine the cause of the blaze, with an update expected on Wednesday morning. It's thought a spark from the light switch ignited the gas.
Unexploded gas bottles were still visible on the outside of the building. Avondale senior station officer Chris Powersaid the leak had come from inside the house.
The man and woman were renting the sleepout. Since it was built the property has changed hands twice.
It's not yet known who installed the kitchen or whether the current property owner was aware the dwelling was illegal.