Wild scenes have erupted outside a Brisbane courthouse after a man was found not guilty of manslaughter over a 2020 Australia Day party fatal stabbing.

Joshua Lindsay Elkerton-Sandy, 21, is set to walk free after a Brisbane Supreme Court jury took only a couple of hours to reach its verdict on Wednesday.

Security was beefed up at the courtroom following tensions between supporters of Elkerton-Sandy and Kane Alexanderson who was 18 when he died of a stab wound to the heart three years ago.

READ MORE: Tensions boil over at Brisbane Australia Day manslaughter trial

Tensions boiled over after the verdict with groups clashing outside the court building despite a heavy security presence.

At one stage Elkerton-Sandy supporters retreated back into the court building, sprinting to evade a rival group in pursuit.

A woman was restrained by a number of security in the court foyer.

Police soon arrived with a number of officers standing outside the court building.

The jury heard Elkerton-Sandy reached for a knife when a group including Alexanderson barged into a Brisbane CBD high-rise apartment three years ago.

Members of the group had a "beef" with Elkerton-Sandy and planned an attack for days, defence barrister Stephen Kissick told the jury during the trial.

A nine-strong group including Alexanderson had been drinking at a building nearby and left to confront Elkerton-Sandy who was at the apartment with friends to watch Australia Day fireworks.

The group snatched a security card for the apartment and seven members were able to barge their way in with Alexanderson leading the charge, the court heard.

Some were armed with beer bottles.

After a confrontation that lasted barely a minute, the group fled with three members suffering stab wounds.

One was slashed in the face, another in the torso and arm while Alexanderson was stabbed twice, including a 6cm deep wound that penetrated his heart.

He died six days later.

"There will be no … contest that Joshua Elkerton-Sandy caused Kane Alexanderson's death," crown prosecutor David Nardone had told the jury.

"The question becomes did Joshua Elkerton-Sandy lawfully act in self-defence?"

Kissick told the jury that Alexanderson was part of an intoxicated mob who had committed a violent home invasion.

He said Elkerton-Sandy had reached for the first thing that he could see when the group entered and grabbed a knife.

"If seven people ran into your house unexpectedly, what would you do?" Kissick asked the jury.

"This wasn't just a bunch of mates popping over … it was a run-in gang bashing.

"He chose to stand up to this gang … he was obviously entitled to defend himself."

Elkerton-Sandy looked skyward in relief when a not guilty verdict was read out.

The public gallery was quiet after Justice Sean Cooper had earlier warned they could not express their feelings about a verdict.

However, groups clashed outside the court building soon afterwards.