Process the wafer biscuits in a food processor until finely ground. Add the butter and pulse until the crumbs are moistened. Pour the mixture into a 23cm springform tin, patting it into the bottom. Bake for 10 minutes or until lightly browned; set aside. Leave the oven on.
Process the cottage cheese in a food processor (you can use the same bowl) for 1 minute or until smooth.
Beat the cream cheese and 1 cup of the sugar with an electric mixer in a medium bowl until light and fluffy. Beat in the whole eggs and egg whites, one at a time, beating well after each addition. Beat in the puréed cottage cheese, the flour, lemon rind, lime rind, lemon juice and lime juice until well blended. Pour the mixture into the prepared tin and bake for 50 minutes or until the outer edge is set; the very centre may still "wiggle" slightly.
Meanwhile, combine the sour cream and the remaining sugar in a small bowl. Spread over the top of the hot cake and bake for a further 5 minutes. Remove from the oven and place the tin on a wire rack to cool. Refrigerate the cheesecake in the tin for at least four hours before serving.