She opened her front door to investigate and almost immediately was overcome by thick, black smoke and menacing flames.

"My god, the hot air came to my face and the black smoke, I couldn't see anything, all I could see was fire coming through the carpet in front of me," she said. "It was everywhere."

A police sergeant and a 20-year-old bystander have emerged as heroes after they miraculously saved Mrs Donati, Susan and Susan's son from near-death in the fire in a Bexley unit block on Monday night.

Unable to leap over the flames that had spread from Susan's unit into the hallway of the small red-brick complex, Mrs Donati grabbed a hammer to break a window onto her balcony to escape.

At 60, she said she was too weak to smash the strong glass and started panicking.

"Then I saw the shadows of a policeman through the glass and I screamed at him, 'break the glass, break the glass', and he smashed a small hole so I could pass the hammer out and then he smashed a bigger hole and pulled me out onto the balcony."

Moments earlier, 20-year-old Isah Makki had been walking along Queen Victoria Street to his uncle's house for dinner when he saw flames shooting out of a window on the third floor and heard Susan and her son screaming for help.

Police and fire crews had not yet arrived so Mr Makki and another bystander acted quickly.

"We just said, 'we've gotta go in'," Mr Makki said.

They kicked in the glass security door on the ground floor and ran up to the apartment to save the pair, who were in a "complete panic".

Susan was screaming out for her pet dog and bird and her son was crying hysterically.

Mr Makki took them both down to the second floor but Susan was insistent on running back upstairs to save her budgerigar and Maltese terrier.

"The smoke was so thick by that stage, I couldn't even see 10 centimetres in front of me," Mr Makki said. "My eyes were burning, I had my jumper covering my face just trying to breathe. It was very ugly."

He ran back up to the third floor with Susan and waited at the door while she braved the smoke and flames to save her animals.

"Every few seconds I'd scream out 'are you OK' and she'd reply 'yes, yes'," he said. "It felt like she was in there for days."

About 30 seconds later, Susan emerged with the blackened pets and they ran out to the front of the complex coughing and spluttering.

A passing police car had stopped at the scene in the meantime and one of the officers saved Mrs Donati.

The sergeant, who then ran back inside the complex to bang down other residents' doors and help them evacuate, was taken to hospital suffering smoke inhalation.

He has since been released and was "having a good sleep" on Tuesday, a police spokeswoman said.

Paramedics also took Susan and her son to hospital with minor burns and smoke inhalation.

Acting Superintendent Wayne Phillips from Fire and Rescue NSW said the fire started when a pan of hot oil and chips caught fire in Susan's kitchen.

She tried to extinguish the fire with water but it only ignited the flames further.

Presumably in a panic, she grabbed the hot pan and ran through the apartment, spilling it on flammable couches and curtains, Acting Superintendent Phillips said.

It then spread to the carpet in the communal hallway.

Thirteen residents had to be evacuated.

"Her and her son were extremely lucky," he said. "The loungeroom and kitchen are all completely damaged by fire and the hallway as well."