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The guilty plea from Stephen Anthony Fernyhough came as more details of the incident emerged in a summary of facts and postmortem report.
The 25-year-old was accused of murdering Ryan John Court at Dunedin on April 27 this year.
In May, he pleaded not guilty to that charge.
Mr Court (35) died after being found unconscious in the toilet area of an inner city bar early that day.
The postmortem examination found ''sudden death in a 35-year-old man following a neck hold''.
Fernyhough appeared at a short hearing in the High Court at Dunedin yesterday, where he appeared calm and attentive to proceedings.
Justice Christian Whata granted a request to amend the charge to manslaughter, and the charge was read to the defendant.
Fernyhough responded: ''Guilty.''
Justice Whata entered the conviction, said prison for the defendant was inevitable, and gave him a three strikes warning.
Fernyhough was remanded in custody and sentencing was set for September.
Defence counsel John Westgate told the court Fernyhough had raised the idea of restorative justice with Mr Court's family, and was ''willing to engage in that process''.
Justice Whata said that was up to the family.
Crown counsel Robin Bates said after the hearing the decision to charge Fernyhough with manslaughter had been made by the Crown when it reviewed the evidence after receiving the postmortem.
The summary of facts said Mr Court had been socialising at Craft Bar in central Dunedin with family and friends.
About 1.30am on the Sunday, the defendant went to the men's toilet of the bar.
''Initially, the conversation was of a light-hearted nature,'' the summary read.
''For an unknown reason the conversation between the pair became more aggressive, which resulted in the defendant and the victim being engaged in a physical altercation.''
During the altercation, the defendant managed to get behind the victim and applied a ''choker-style hold'' for between 20 and 40 seconds, resulting in the victim very quickly becoming unconscious.
Mr Court fell to the ground when released, and Fernyhough left the toilet, uttering a comment similar to ''Don't mess with the wee man''.
Mr Court received first aid, but was pronounced dead shortly after by ambulance staff.
Fernyhough left with associates, and became involved in a second altercation with a man at the Brimstone bar in Princes St, resulting in a window being broken.
There was no plea made on a second charge of intentionally damaging a window.
The defendant was arrested, interviewed about 8am, and admitted placing Mr Court in a headlock.
''He stated that he did so to protect himself and that he did not intend to kill the victim.''
The postmortem report noted findings in a death following a neck hold ''may be minimal'', and they were in this case.
Compression of the carotid arteries on the side of the neck could impair blood flow to the brain, with death resulting from carotid sinus stimulation causing a neural reflex that slows or stops the heart.
The autopsy also said Mr Court fell to the floor face first ''without attempting to protect himself from the fall''. No other injuries were identified that contributed to the cause of death.
Mr Court's family chose not to comment on the guilty plea.