Preheat oven to 350 degrees F (175 degrees C). Grease 2 loaf pans.

Beat eggs and sugar together in a large bowl with an electric mixer until fluffy. Sift 2 1/2 cups flour and baking powder together in a separate bowl. Beat half of the flour mixture gradually into the egg mixture with electric mixer on low speed. Mix in orange juice and vegetable oil. Add remaining half of the flour mixture, orange zest, and anise seed; beat until smooth batter forms.

Pour boiling water over the sultana raisins, drain, and pat completely dry with a paper towel. Mix walnuts and raisins in a bowl with 1 tablespoon flour; fold into batter. Spread batter evenly into prepared loaf pans.

Bake in preheated oven until a toothpick inserted into the center comes out clean, about 60 minutes. If cake top browns before cake is fully cooked, cover with aluminum foil and bake until set, about 10 minutes more.

Cool in the pans for 5 minutes before removing to cool completely on a wire rack. Dust with confectioners' sugar.