"It rocked the house."
He said "lots of trucks and police" turned up near his Bayswater Avenue home soon after the explosion.
His house lost power but electricity was restored by this afternoon.
He said staff were very rarely staff at the substation.
Middlemore Hospital spokeswoman Lauren Young said this afternoon the man was in an intensive care unit, still in a critical condition.
A spokeswoman for Vector confirmed an incident happened on the company's northern network, causing injury to one of Vector's Electrix field service staff.
"Our thoughts are with him, his family and his co-workers," she said.
Earlier today, Vector said the outage was believed to be a feeder fault.
The substation appeared to have two transformers which changed voltage levels from high incoming transmission voltages to lower distribution voltages.
Vector did not say specifically if a circuit breaker or transformer blew up.
The company has not said if the fault was the outcome of the man's accident, or if the man there to investigate an existing fault.
In February, more than 3,200 homes in nearby North Shore suburbs Milford and Takapuna lost power after a feeder fault.
On its website, Electrix described itself as a "leading provider of engineering, construction and maintenance services to asset owners in the utility, industrial, commercial, resources, and infrastructure sectors".
