Witness Esau Marshall was cross-examined on Friday by defence counsel Jon Tippett, QC, and said he saw the fight between Liam Jurrah and his cousin Basil Jurrah at the Little Sisters Camp in Alice Springs last March.
Asked if he had really seen the footballer with a machete over his head smashing down on Basil Jurrah and using the sharp edge of the weapon, Mr Marshall said he had.
"He was celebrating. He was saying 'BJ, BJ, BJ'," Mr Marshall said.
BJ is the nickname of Basil Jurrah.
"This was something you watched?" Mr Tippett asked.
"Yes," Mr Marshall replied.
Another witness, Alan Collins, also testified that he saw the footballer hitting Basil Jurrah with a machete.
Asked how many times Jurrah had hit his cousin on the head Mr Collins said, "many times, and his nose as well".
He was challenged by Mr Tippett as to whether he actually saw the incident or only heard about it later.
Mr Collins said he had been running away from the fight but turned around and saw the footballer and Basil Jurrah.
The Supreme Court hearing, which began in Alice Springs on Monday, had earlier been told that Liam Jurrah was involved in two violent incidents at Little Sisters on March 7 last year.
The court was told that the conflict relates to a dispute between two families in the outback town of Yuendumu, where Jurrah is from.
The court has been told that Basil Jurrah suffered a fracture to his skull, fractures to his face including his eye socket and nose, and four scalp lacerations.
The trial continues on Monday with a video expected to be shown of Liam Jurrah giving his statement to police.
