Pre-heat oven to 180°C (160°C fan-forced). Grease and line the base and sides of a 10 x 20cm loaf pan.
Combine the flours, baking powder and mixed spice in a large mixing bowl.
In a separate bowl whisk together the oil, sugar, eggs, milk and vanilla. Mix in the banana. Add to the bowl of dry ingredients and mix until just combined. Pour into the prepared pan and bake for 40 minutes until golden and an inserted skewer comes out clean.