Keen to celebrate Australia Day, Joshua Lindsay Elkerton-Sandy and some friends booked an apartment in Brisbane's CBD.
He did not realise until later that evening a group of "enemy friends" had the same idea and were staying in a nearby high-rise building.
After yelling and swearing from their respective balconies, seven members of the rival group entered Elkerton-Sandy's apartment and a fight erupted, Brisbane Supreme Court has heard.
Three minutes later a member of the group, 18-year-old Kane Alexanderson, was back downstairs awaiting an ambulance nursing two stab wounds.
He died six days later.
"The question in this trial very much revolves around the question of self defence," crown prosecutor David Nardone told the jury.
Mr Nardone said there was little doubt that Mr Alexanderson and his group were "up to no good" when they left their high-rise apartment and headed towards Elkerton-Sandy's party on Australia Day in 2020.
"They seem committed to confront and even assault the accused (Elkerton-Sandy)," he said.
After gaining entry to Elkerton-Sandy's apartment there was an exchange of words and they started to fight.
"But the group ... didn't know or didn't see that the accused was armed with a knife," Mr Nardone said.
Elkerton-Sandy, 21, is accused of stabbing three people including Mr Alexanderson twice.
When the group became aware of the knife they fled with the altercation lasting little over a minute, the court was told.
One group member was slashed on the face, another on the torso.
Mr Alexanderson suffered a six-centimetre stab wound that penetrated his heart which caused internal bleeding, leading to cardiac arrest and he died on February 1.
Elkerton-Sandy told police that he was defending himself.
He said he did not realise that he had booked an apartment across from Mr Alexanderson's group until they started swearing at him from the balcony, the court heard.
They yelled "have a go, don't be a bitch, we'll come up there and stab you" and were egging him on with Facebook posts, he told police.
Elkerton-Sandy told police that Mr Alexanderson's group burst in, smashed bottles over his head and pinned him against the balcony.
He claimed he was "half hanging over" the balcony and thought he was going to die before his girlfriend grabbed him, court heard.
In one police interview Elkerton-Sandy claimed Mr Alexanderson was accidentally stabbed by a member of his own group, Mr Nardone said.
In another he said a friend stabbed Mr Alexanderson and the others to stop them kicking him while he was lying on the floor.
He gave another account to an undercover officer in a watch-house cell, Mr Nardone said.
Elkerton-Sandy told the undercover officer that he knifed all three when they came at him in the apartment including one he knew as Kane who he stabbed "twice in the guts".
Elkerton-Sandy has pleaded not guilty to one count of manslaughter.
The trial before Justice Sean Cooper continues.
