A man arrested after the body of a well-respected community health nurse was found inside his Sydney unit wouldn't understand what he has allegedly done, friends say.
NSW Police were called to the home on Nicholson Street in Balmain East on Thursday afternoon after concerns were raised for the welfare of those inside.
On arrival, officers found the body of a 62-year-old male community mental health nurse.
A 40-year-old man - identified by Sydney media as Peter Kemball - was arrested and is in hospital being treated for lacerations.
It has been reported Mr Kemball suffers from mental health problems.
Police confirmed the two men were known to each other.
Mr Kemball's friend, Jane Stainthorpe, said he wouldn't have killed someone "without it being a brain snap".
"Some days he could be the nicest person you've ever met, other days he was the complete opposite," Ms Stainthorpe told Nine News.
"He would not understand what he's done at the moment."
NSW Nurses and Midwives' Association said the caseworker was an advocate for improvements in mental health.
"All healthcare workers, regardless of their chosen speciality or field of work, are incredibly resilient individuals - none of them ever go to work not expecting to return home," association general secretary Brett Holmes said in a statement on Friday.
"Our immediate thoughts continue to be with our member's partner, family, friends and co-workers at this difficult time.
"He was a very well respected mental health nurse, a mentor to his nursing colleagues and an advocate for improvements in mental health."
Sydney Local Health District chief executive Dr Teresa Anderson informed staff with "much sadness" of the "tragic death of one of our community mental health colleagues".
"This is a terrible loss for his partner, family and loved ones and is (a) devastating loss for us all," Dr Anderson wrote in an email.
Health Minister Brad Hazzard and Mental Health Minister Bronnie Taylor issued a joint statement on Thursday evening following the incident.
"We and the NSW Health family are devastated by the tragedy that has fallen upon one of our committed health care workers," the ministers said.
"We extend our deepest sympathy to the family, friends, and co-workers of the victim."