A firefighter who rescued an unconscious toddler from a burning Invercargill house says that in another 30 seconds he would have been retrieving a body.

Deane Chalmers, 37, entered the house, which was half engulfed by flames, shortly after 8am yesterday and had to use his touch to locate the 3-year-old boy because of the thick smoke inside.

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. "And probably even closer to that - I hate to think how much closer he was [to death]. It was very touch and go."

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The boy was flown to Middlemore Hospital in Auckland in a critical condition.

Three other people were also taken to hospital as a result of the fire. Two were treated for smoke inhalation and another for minor injuries.

It took about 20 minutes to bring the fire under control. The cause of the blaze is under investigation.