Hei Hei resident Jenee Pullen (34) was watching a movie with 1-year-old son TJ Roberts with the curtains pulled to block the 30C heat when the explosions happened across the road at the station.
"We just heard bang bang bang," she said.
"I thought a passing truck had dropped something.
"But when I had a look outside saw smoke and fireballs. It was a pretty big fire.
"You could feel the heat from the flames."
Unsure what to do, she then heard someone outside yell, "It's gonna blow!"
She rushed outside to ask police and Transpower officials if she should evacuate.
They reassured her that the transformer was protected by blast walls and that the blaze was contained.
"It seems under control now," she said.
A fire service spokesman confirmed the blaze at Transpower's Islington Substation on the city's Roberts Rd was an oil fire.
The fire was extinguished by 3pm.
The billowing dark grey smoke turned to white clouds before clearing altogether, within minutes.
The Roberts Rd cordon has now also been lifted.
Shift manager Brett Dunn said the fire had produced a lot of smoke, but was in a "secure location" meaning it was unlikely to spread.
Fire crews were brought in from throughout the city to help battle the blaze using foam. There were seven fire crew at the scene.
Mr Dunn said there have been no reports of injuries.
The fire was called in by multiple members of the public about 1.30pm, and there had been reports of an explosion, Mr Dunn said.
The fire follows a 5.7 magnitude earthquake that struck the city yesterday.
A Transpower spokeswoman said the fire had broken out in an enclosed area, which was immediately isolated to prevent further damage to the substation.
She said power supply had not been affected by the fire, and Transpower staff were at the scene.
The company would be investigating, but the spokeswoman was unable to say what may have sparked the fire.
Edward O'Driscoll saw black smoke billowing from the substation about 1.30pm, as he was driving along Christchurch's Main South Rd.
He said other witnesses had reported hearing an explosion.
He did not see flames and said by about 2pm he could no longer see smoke in the area.
"Traffic was backed up, I think there were quite a few sight seers watching," Mr O'Driscoll said.
