Preheat oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large bowl, whisk the warm milk and butter until the butter is fully melted.
In a small bowl, mix 3 tablespoons of the warm milk with custard powder until smooth. Add this to the milk mixture and whisk to combine.
In a separate bowl, whisk together eggs, sugar, cinnamon, nutmeg, and vanilla.
Combine the egg mixture with the milk mixture and whisk thoroughly.
Add the bread pieces, bananas, and raisins to a large bowl. Pour the custard over and mix gently, pressing to ensure the bread absorbs the liquid.
Cover and let rest for 20 minutes.
Transfer to prepared dish and bake for 50–55 minutes, or until golden and just set.
Let rest 5 minutes before serving. Serve warm with caramel sauce or coconut ice cream.