Put the water and sugar in a medium saucepan. Cook, stirring every now and then, on high heat until the sugar dissolves and the syrup comes to the boil. Reduce the heat to medium-low and simmer for 20 minutes, or until the syrup thickens slightly. Set aside to cool.
Juice the oranges and lemons and strain the juice into the sugar syrup. Add the passionfruit pulp and mix well. Pour the syrup into a sterilised bottle and chill in the refrigerator overnight. (The syrup will keep in a sterilised bottle in the refrigerator for a week.)
When you are almost ready to serve, peel, core and finely chop the pineapple and put the fruit in a punch bowl or jug. Add the passionfruit syrup, orangeade and lemonade. Stir well. Add some fresh mint leaves and ladle or pour into glasses half-filled with crushed ice.