In a Facebook comment Venter said Luvuno completed the 180km cycle leg but missed the cut-off time by about five minutes.
"He fell in the transition area after the swim, broke a bone in his hand and lost about 10 minutes while his hand was being strapped," Venter said.
In January, Luvuno completed the Ironman 70.3, also known as the half Ironman, with the full race as his next goal.
Luvuno inspired many when he completed the Comrades Marathon in 2018. His difficult background makes him even more special to many of his supporters.
He dropped out of school as a teen and turned to crime, which landed him in prison. He had a leg amputated in 2009 due to cancer. When Luvuno and Venter met in Centurion in 2016, Luvuno was homeless and battling substance abuse.