Dr Douglas Scott, a heart specialist who lives near where the crash happened, was one of the first at the scene and said there was no chance of the teenager surviving the crash.
He said there was nothing he could do to help Mr Sionepulu.
"[He] was unconscious, so I knew his injuries were unsurvivable,'' Dr Scott said.
"The rod went just under his heart. It was horrible ... a freak accident.
"It was a terrible situation. He wasn't conscious, otherwise I would have tried to comfort him [until he died].''
Family and friends of Mr Sionepulu, who lived in Panmure and worked at KFC, posted tributes to him online.
"R.I.P my lil brother DavidLatanoa Sionepulu..Love you always, you will forever be in my heart right..,'' Johnny N Melzie Savage-Oleiki wrote.
"You're bloody too young to be taken away from us so soon,'' said Lupe Vakalahi.
Staff at KFC Glen Innes, where Mr Sionepulu had worked for two-and-a-half-years, were shocked by news of his death.
"He was a very reliable, committed and hard working team member and he will be missed,'' KFC general manager Brent Kitto said.
Counselling services have been offered to staff, he said.
Mr Sionepulu was the owner of the car.