1.Grease 20cm x 30cm (8-inch x 12-inch) rectangular pan; line base and long sides with baking paper, extending paper 5cm (2 inches) over sides.
2.Process 185g (6 ounces) of the biscuits until fine; chop remaining biscuits coarsely.
3.Stir condensed milk and butter in small saucepan over medium heat until smooth.
4.Combine processed and chopped biscuits, rind, juice and coconut in medium bowl. Add condensed milk mixture; stir to combine. Press mixture firmly into pan. Refrigerate 30 minutes or until firm.
5.Meanwhile, make lime icing. Sift icing sugar into small heatproof bowl; stir in butter, juice and enough water to make a thick paste. Place bowl over small saucepan of simmering water, stir until icing is spreadable.
6.Spread icing over slice. Refrigerate 30 minutes or until firm.