Take 1 of the lemon grass stalks and remove the outer leaves. Finely slice it and place it in a bowl along with the sweet chilli sauce, coriander and oil. Place the prawns in this marinade, cover and leave in the refrigerator to marinate for 1 hour or overnight.
Remove the prawns from the marinade. Take the remaining 4 lemon grass stalks and remove a few of the outer layers to give you a thin lemon grass skewer. Make a hole through each prawn at its thickest part using a metal skewer, then thread 5 of the prawns on to a lemon grass stalk. Repeat with the remaining lemon grass and prawns.
Place the prawn skewers on a barbecue and cook for 4 minutes on each side, or until the prawns have turned pink and are firm to the touch.
Serve the prawns straight from the barbecue with sweet chilli sauce for dipping.