A man accused of causing his cousin a traumatic brain injury says he feared his relative would continue attacking him when he threw the single punch.
Shane Vivian Smith, 36, of Ravenswood has pleaded not guilty to assault on the basis of self defense.
Mr Smith, who took the stand this morning, was asked to leave his uncle's house in Ravenswood following a drink session last year.
He claims he was pursued by his cousin outside who then punched him twice over a disagreement.
Mr Smith said he returned a punch as a reflex and in self-defence.
The alleged victim is said to have suffered significant brain injuries and had to be flown to HobArt for treatment.
The crown has accused Mr Smith of being the aggressor who punched his cousin without provocation stating the defendant had a "Jekyll and Hyde personality".