Preheat oven to 180C/350F. Grease three 8cm x 16cm (3 inch x 6 inch) (top measurement) loaf pans; line bases and long sides with baking paper.
Sift flour, baking powder, bi-carb soda, spices and sugar into a large bowl. Fold in desiccated coconut, pineapple, banana, egg and oil until combined.
Divide mixture between pans; place on oven tray. Bake for about 45 minutes. Cool cakes in pans on a wire rack.
Meanwhile, make lemon icing. Sift icing sugar into a small heatproof bowl. Stir in butter and enough of the juice to make a thick paste. Place over a small saucepan of simmering water; stir until icing is of a pourable consistency.