Cook sweet potatoes in boiling water until tender, 20 to 30 minutes; drain. Place sweet potatoes in a bowl and mash until smooth.
Preheat oven to 180 C / Gas 4. Grease a 900g loaf tin.
Stir sugar and oil together in a bowl until well mixed; beat in banana. Combine flour, bicarb, cinnamon, nutmeg, salt and baking powder in a bowl. Alternate stirring the flour mixture and the 80ml water into the sugar mixture until combined. Mix in sweet potatoes and pecans; stir until mixture is smooth. Pour into the prepared loaf tin.
Bake in the preheated oven until top is golden brown, about 1 hour.