Mr Brosnan, a manager-technician for JAE Services Alexandra, had been called by an insurance company to the Marslin St house to clean blood from the carpet after 86-year-old Ethel Watt had an accident on Tuesday.
Just before lunch-time on Wednesday, he went outside then heard the smoke alarm activate in the house, and the woman began screaming.
Mr Brosnan raced into the house and saw Mrs Watt in the kitchen with her clothes on fire. He ran into the bedroom and grabbed some blankets to put over her and tried to suffocate the flames.
He called out to neighbours for help.
"I threw the phone at them and told them to call emergency services."
He then went to get more blankets to fight the flames.
The fire started spreading through the kitchen so Mr Brosnan tried to put out the flames with water.
The episode took not much more than a minute, he said.
He began to feel the effects of the toxic fumes and remembered lessons from his first aid training.
"The first rule was to secure the scene and make yourself safe so I went to get some air. The Fire Service arrived followed shortly after by the ambulance."
Ambulance officers gave him oxygen.
Mr Brosnan said he had never had to deal with anything like that before and was glad he kept up his First Aid courses with St John every two years.
"I was also very lucky I had a woollen jersey on so I didn't catch fire as well."
The woman was wearing synthetic clothing, he said.
Mrs Watt's daughter contacted him on Wednesday night to thank him for everything he had done in trying to save her mother's life, he said.
Mr Brosnan received burns to both hands, and his hair and eyebrows were singed.
He had been going over and over his actions throughout the night wondering if there was anything further he could have done to save Mrs Watt.
"The Fire Service and ambulance said I had done everything right, but you always wonder if there was perhaps some little thing that would have seen a better outcome."
Detective Sergeant Derek Shaw, of Alexandra, praised Mr Brosnan's efforts in trying to save Mrs Watt.
Her body was removed from her home on Wednesday evening and taken to Dunedin for a postmortem.
Det Sgt Shaw said the house had been opened to the family although ongoing investigations as to the cause of the fire would continue.
He could not comment on the cause of death, saying that would be a matter for the coroner.
Gas engineers and clothing experts will be involved in the investigation, he said.
