Place the strawberries, blueberries, raspberries and kiwi fruit in a blender with 1 cup of the orange juice, and blend for about 45 seconds, or until smooth.
Using a large spoon, push the berry mixture through a sieve, discarding the solids. (see notes below).
Crush the ice in the blender (use the ice crusher button on the blender if it has one); add the sieved berry purée, the remaining orange juice and the sugar; blend to combine, then taste for sweetness and add more sugar. If you want your drink slightly thicker, use slightly less orange juice.
Pour into 6 glasses and top each with a sprig of mint, or pour into a serving jug, and serve immediately.
Notes: The original recipe suggests using 12 or more ice cubes if using fresh berries; and that if you are using frozen berries that you may prefer not to use ice cubes at all. My blender does not have an ice crushing facility, and I actually chose to use frozen berries partly because I didn't want to cause unnecessary strain on the blender! Secondly, I was making this just for myself, so the quantity I made was going to last for several days: after the ice had melted, the juice would probably have been too watery for my taste. And thirdly, the weather was just not hot enough for me to fancy SUCH a cold drink. Note on step two: I wasn't very happy about completely discarding the solids, so I have put them in the freezer while I think of where I can use them!