The person was one of only two general psychiatrists at the hospital and an anonymous health worker said the assault on Monday put more pressure on an already understaffed system.
The health worker, who did not wish to be named, told the Otago Daily Times the psychiatrist was seriously assaulted in the general adult inpatient ward.
A police spokeswoman said the assault was being investigated, and inquiries were ongoing to determine the circumstances.
"One person was hospitalised with serious injuries."
Te Whatu Ora Health New Zealand (HNZ) Southern Group director of operations Hamish Brown said a serious incident had occurred involving a staff member.
"We all deserve to feel safe in our working environment and we are devastated that a colleague has been injured while at work.
"We are continuing to closely support our colleague and our staff."
HNZ could not comment further on the incident as police were investigating, he said.
Southland Mental Health Services had two FTE general psychiatrists.
The service had other specialist psychiatrists, nurse practitioners and locum psychiatrist support from across the country.
"Our Dunedin mental health services team continue to provide support to Southland as well."
The shortage of psychiatrists in the Southern area was one of the issues raised by doctors in September amid nationwide strikes, as reported by the ODT.
A Dunedin-based psychiatrist said staff numbers in the Otago area had not increased in 30 years, although the population had roughly doubled.
Invercargill had just two psychiatrists, he said at the time.
A colleague had been travelling to Invercargill to help out, and this put more pressure on Dunedin.
