A manhunt is under way after a young female staff member was found dead in the gymnasium at a private school in Sydney’s city centre.

Police said on Thursday they were looking for a male employee from St Andrew’s Cathedral School after the woman was discovered with serious head injuries inside the gym toilets.

Her name has not been officially released, but it is being widely reported the dead woman was the school’s 22-year-old water polo coach Lilie James.

Emergency services were called to the school about midnight after receiving reports of concerns for the woman’s welfare.

Superintendent Martin Fileman said police found a “confronting” scene when they arrived, due to the severity of James’ injuries.

A crime scene was also set up near a coastal reserve at Vaucluse, in Sydney’s eastern suburbs, where investigators found items linked to the killing.

Police helicopters and boats were patrolling the coastline off South Head on Thursday morning. Fileman said detectives had been looking into the possibility the missing man had taken his own life.

The man has been named as Paul Thijssen, a sports assistant at the school – and a former St Andrew’s student.

“We’re currently looking for a male person also in his early 20s who we believe can assist us … we believe he was also employed by the school,” Fileman said.

“That male person is currently outstanding.”

Thijssen and James were known to each other but Fileman said there was no police history of domestic violence. Investigators were still working to establish the nature of their relationship, he said.

Paul Thijssen was a former student of the school, who worked there as a sports coach. Photo: LinkedIn

He said police were interviewing several people, including other staff who were in the building at the time. Detectives are also combing the school’s CCTV footage to piece together what led to the killing.

Fileman said the circumstances of James’ death were confronting for students as it involved school staff.

“We’re currently working in with the school to ensure the welfare of the students and they have measures in place to make sure those students are taken care of,” he said.

The 138-year-old independent Anglican school school will be closed to all students until next week, except for those sitting year 12 exams.

“An area of the school, not accessible to students, is a crime zone, and we await the approval of police before bringing everyone back on site,” a school spokesperson said.

“The only students onsite are those sitting exams, and we ask that the media does not seek to interrupt their concentration or state of mind as they sit exams that are significant for their graduation.”

St Andrew’s Cathedral School has more than 1400 students.

Police and the school said there was not believed to be any ongoing threat to students or the wider public after the attack.

Counsellors will be on site for parents and students as required.

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-with AAP