In a bowl, mix everything together except for the sour cream. Place kangaroo fillets in mixture and coat well. Cover the bowl with cling wrap and let marinate for at least 4 hours, turning fillets every so often. When first coming up with this recipe I marinated mine for 14 hours :).
After marinating. Melt some butter in a hot frying pan and cook fillets for about 8 minutes on each side or to your preferred cookedness. (Depending also on the thickness of the fillets).
Once fillets have cooked, pour the remaining marinade into the frying pan and bring to the boil, stirring constantly. Add the sour cream and mix well. Salt and pepper to taste and spoon over the kangaroo fillets.