"The whole house was up in flames," he said.
"It's frightening how fast and big it was."
Dozens of onlookers watched as firefighters in breathing apparatus worked to fully extinguish the fire, after they managed to bring it under control.
Willowbank Station Officer Peter O'Shea, who was working overtime at the St Kilda station yesterday, was one of the first firefighters at the scene.
He found a "pretty involved fire", and the occupants of the house were all outside.
As soon as firefighters were able to get into the house, they found the occupants' dog under a bed.
"We tried CPR and oxygen on the dog, but sadly it didn't work," Mr O'Shea said.
The occupants were "very distressed and upset", he said.
Their cat was found alive and well at a neighbouring property.
A Fire and Emergency NZ spokesman said the single-storey building, which comprised two separate units, was "well involved" when firefighters arrived at the scene.
"All people are accounted for and there are no injuries," he said.