He rushed across the street to find windows blown out.
The only person inside the house had a minor hand injury, he said.
A Fire and Emergency NZ spokesman said "the occupant was cooking with portable gas and the accident happened".
The workman said he saw a damaged ceramic-top stove "with half of it missing, exploded". He also saw what appeared to be gas canisters.
"He's bloody lucky by the looks of it," he said.
A neighbour said she got "a big fright" from the blast.
Locals say it's a quiet street but the occupants of the damaged house are largely unknown.
"You just never see them," one neighbour said.
Detective sergeant Craig Johnson said it was too early to know what caused the explosion.
He wouldn't confirm if a portable gas cooker had been involved.
"We will be here for some time and we will be cordoning this address," he said.
The spokesperson said two crews were in attendance and the fire had been extinguished.
Opawa is an inner suburb of Christchurch, 2.5km south-east of the city centre.