Preheat oven to 200C. Grease an 8 cup muffin tray or line with paper muffin liners.
In a large bowl, combine flour, sugar, salt and baking powder. Add vegetable oil, egg and milk; mix well. Fold in diced peach and nectarine. Fill each muffin cup to the top with muffin mix. Sprinkle a little brown sugar onto the top of each uncooked muffin.
Bake in preheated oven for 18 to 20 minutes. Check muffins regularly after 15 minutes of baking. Serve warm or cool.