Preheat oven to 175 degrees C. Grease a 22cm x 32cm baking dish.
Combine flour, butter, and icing sugar in a bowl. Stir in macadamia nuts. Press half of the mixture into a 1.5cm-thick layer on the bottom of the prepared dish to make the crust.
Bake in the oven until lightly browned, 15 to 20 minutes.
Meanwhile, stir mangoes, white sugar, 1/3 cup water and lemon juice together in a saucepan. Cook over medium heat until mangoes reduce by one-third, about 7 to 10 minutes.
Combine cornflour and 3 tablespoons water in a small bowl until smooth. Pour into the mango mixture; cook until thickened, about 5 to 7 minutes. Cool briefly and pour over the crust. Sprinkle the remaining flour mixture on top.
Continue baking until lightly browned, 20 to 25 minutes. Cool until set, about 15 minutes. Cut into bars.