Paramedics have assessed 15 patients at Villawood Detention Centre as detainees were forced to jump out of a second-storey window after a fire broke out on July 6.
More than 40 firefighters were called to the centre in Sydney's west about 11.20am.
Three aerial pumpers, specialist HAZMAT and rescue crews were deployed to the incident.
"During the early stages of the fire, three detainees on the top floor smashed windows and jumped onto mattresses below in an effort to escape the smoke," a Fire and Rescue NSW spokesperson said.
"They were assessed at the scene by NSW Ambulance paramedics, as were 12 staff who sustained smoke inhalation as a result of the incident.
"[Firefighters] are now pulling apart the ceiling within the compound to ensure it has not spread into the roof void."
One person has shared a TikTok video from inside the centre.
Smoke can be seen inside billowing from the building and an emergency alarm sounding before detainees are directed to kick out a window and jump to the ground.
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A spokesperson for the Australian Border Force said it was aware of the incident and emergency services had been notified and were responding accordingly.
"All staff, service providers and detainees have been accounted for," the spokesperson said.
"As this incident is subject to ongoing investigation, it would not be appropriate to comment further at this time."