The manager of a troubled Haymarket apartment building has raised concerns about overcrowding in the complex after a man allegedly doused his partner with flammable liquid and set her alight.

The 34-year-old female victim is in a serious but stable condition at Concord Hospital after sustaining burns to her head and upper body.

The incident is believed to have occurred in a common area on level six of the Regis Towers complex on Castleraegh Street in Sydney's CBD, where the woman is understood to have been living as a subtenant in an apartment in the building.

Neighbours found the woman screaming for her life and covered in flames in the sixth floor lobby. One resident smothered the flames with a gym towel before security guards and emergency services rushed to the scene.