Wash oranges and put in a saucepan with water just enough to cover. Put the pan on medium heat and boil them for 30 minutes. Leave to cool a little.
Discard the calyx (green stem bit on top) and pips of the oranges. Put rest of the whole oranges, cut in chunks in the blender or food processor. Process until the oranges become smooth puree.
Pre-heat the oven 170°C. Line the cake tin with baking paper.
In a large bowl, beat the egg whites until it forms peaks. Add sugar and continue to beat until it becomes a soft meringue.