Police were seen at a property in Heriot Row at about 8am.
A man living at the property said he was woken early by the blaring of a smoke alarm.
However, he said he could hear screaming coming from the shared living space, and he was too scared to leave his room.
When he walked into the shared living space, there were pools of blood all over the house leading outside, he said.
"[I} didn't know he . . . could do something like that," he said about the alleged perpetrator.
The victim left a trail of blood that could be seen leading from the property, down the street, across the intersection to the top of nearby Scotland St.
They were found by police and taken by Hato Hone St John to Dunedin Hospital in a critical condition.
Officers at the scene arrested and walked the alleged perpetrator, who was known to the victim, to the back of a patrol car shortly after arriving at about 8am.
Police later said they were not looking for anyone else in relation to the incident.
Inquiries are ongoing to determine exactly what has occurred, Detective Sergeant Matthew Harris, of Dunedin, said in a statement on Saturday afternoon.
"Police would like to hear from anyone who may have information about the incident, or who has CCTV or dashcam footage of the Heriot Row and Scotland St areas between 7.30am and 9am this morning [Saturday].
"We would especially like to speak with members of the public who stopped to assist the victim before emergency services arrived.
"If this was you, or you can assist with our investigation, please contact 105 – either online or over the phone – and reference file number 260425/4875."
A Fire and Emergency New Zealand spokeswoman said shortly after, crews from Roslyn and St Kilda arrived after police called them due to smoke showing from the property.
After liaising with police, they entered the property and extinguished a small fire inside the property, she said.
- Do you know more? Email reporters@odt.co.nz
— additional reporting ODT Online
