Preheat oven to 180 degrees C. Grease one 23x13cm loaf tin.
In a small saucepan, combine the sultanas and the rum. Cook over medium heat until simmering, then remove from the heat and allow to cool 10 minutes.
In a large bowl, mix together the mashed bananas, brown sugar, canola oil and eggs with an electric mixer for 1 minute. Sift together the plain flour, wholemeal flour, baking powder, bicarb soda, salt and ground cardamom; stir into the egg mixture with a spoon until well blended, Stir in the walnuts and the rum soaked raisins last. Pour the batter into the prepared tin.
Bake in the preheated oven for 50 to 60 minutes, or until a toothpick inserted comes out clean. Allow bread to cool slightly before removing from the tin.