A mattress fire in a boarded-up unit at Hamilton South sparked an emergency response on Friday evening.
Firefighters, police and paramedics rushed to the scene on Fowler Street, where they found a second-storey unit, understood to be unoccupied, had caught fire.
Smoke billowing from the apartment triggered an evacuation of about 20 residents and firefighters worked to gain access and confirm no one was still inside.
Paramedics saw three people on the scene, including a primary school-aged boy, but no one was seriously injured or required hospital treatment, a spokeswoman for the NSW Ambulance told the Newcastle Herald.
Firefighters contained the damage to the unit, a police spokesman said, and extinguished the fire just before 9pm.
Police had secured the building into the evening, and were expected to return at the weekend to continue their inquiries.
