Experienced firefighter Deane Chalmers (37) entered the house, which was half-engulfed byflames shortly after 8am yesterday, and had to feel his way to locate the boy, because of thick smoke.
When he and a fellow firefighter came across the boy lying on the floor of a bedroom, he was unresponsive.
"I think it's fair to say that in another 30 seconds he wouldn't have been alive when we removed him from the property," Mr Chalmers said.
"It was very touch and go."
Mr Chalmers carried the limp child out of the house to safety.
His breathing was "very, very shallow".
The boy was rushed to Southland Hospital in a critical condition after suffering from smoke inhalation.
He was later transferred to Auckland.
Dealing with a child in such a situation was a "firefighter's worst nightmare", said Mr Chalmers, a father of a 7-year-old boy.
When he and two other firefighters equipped with breathing apparatus went in to search for the boy, it was unclear exactly where he was in the house.
Working in trained search patterns, Mr Chalmers and another firefighter felt around the smoke-filled bedroom in opposite directions until they met in the middle, where they found the boy.
"At the point we found him, there was just a very small clearing in the smoke, which allowed us to identify this is the young boy that we are looking for, grab him and get him out of the room as quickly as possible."
Whatever the outcome, Mr Chalmers said he and his fellow firefighters felt they had given the boy a chance.
No further information about his condition was available last night.
Three adults were also taken to Southland Hospital and treated for minor smoke inhalation and cuts.
Southland fire area manager Bruce Stubbs said it took about 20 minutes to bring the fire under control, but firefighters remained at the house for three hours to dampen hot spots.
Fire safety officers believed the blaze started in the lounge area, but had not identified a cause.
