According to L'Equipe, Riebel was helping teammate Erwan Brenterch back to the peloton on the penultimate stage of the Tour after a mechanical problem when he struck an ambulance coming in the opposite direction on the descent of the Pirogue Pass in Paita, Noumea.
Riebel died instantly. Brenterch also suffered a broken leg in the tragic incident.
"It's awful," New Caledonia cycling committee told L'Equipe. "A kid of 20 years... The Tour is over.
"Tomorrow, the peloton will make a tour of Mont-Dore without a race to pay tribute. He was so happy to be here."