The 84-year-old woman returned to the unit on Bobbin Head Rd in North Turramurra at 12.25pm on Saturday “when she located an unknown male in her bedroom”, police said.
“The man then seriously assaulted the woman, before he fled out the front door of the premises.”
The leafy suburb is home to a large number of retirement villages and aged care facilities.
Emergency services were called to treat the injured woman and a crime scene was established.
She was taken to Sydney Adventist Hospital in Wahroonga with multiple serious injuries and lacerations to her face, chest and arm, police said.
She remained in a serious but stable condition early on Sunday morning.
Anyone with information is urged to come forward to police including those with dashcam footage of the area or who may have observed something suspicious at the time of the assault.
There is no physical description of the man available at present.
Officers from Kuring-Gai Police Area Command are investigating with Detective Chief Inspector Luke Spurr to address the media about the incident later on Sunday.