1.Grease 24cm round, 2cm deep loose-based flan tin.
2.Blend or process biscuits until fine. Transfer to medium bowl; stir in melted butter.
3.Press biscuit mixture evenly over base and side of tin, place on oven tray; refrigerate 30 minutes.
4.In a medium saucepan, combine cornflour and ½ cup of the sugar; gradually stir in juices and the water until smooth. Stir over high heat until mixture boils and thickens. Reduce heat; simmer, stirring, 1 minute. Remove from heat; stir in extra butter, egg yolks and rind until butter melts. Cool 10 minutes.
5.Spread filling over biscuit base, cover; refrigerate 2 hours.
6.Preheat oven to 240°C.
7.In a small bowl, beat egg whites with electric mixer until soft peaks form; gradually add remaining sugar, 1 tablespoon at a time, beating until sugar dissolves between additions.
8.Roughen surface of filling with a fork before piping with meringue. Spoon mixture into piping bag fitted with 1.5cm plain tube. Pipe meringue over filling. Bake about 5 minutes or until meringue is browned lightly.
Finely grate the rind from citrus before juicing. The base and filling can be made and assembled up to a day in advance. Meringue topping is best made just before serving. Roughing the surface of the filling will help the meringue to cling to the filling.