Preheat oven to 170 °C. Line a muffin tin with muffin liners.
Toss blueberries in a teaspoon of plain flour and set aside.
To make the streusel mix flour, sugars, nutmeg, salt and cinnamon in a small bowl. Pour in melted butter and mix with a fork until crumbs form. Chill in the fridge while you make the muffin batter.
In a large mixing bowl, add flour, sugar, baking powder, and salt. Whisk to combine. Set aside
To a jug, add buttermilk, oil, eggs and vanilla extract. Whisk to combine.
Add the wet ingredients to the dry ingredients and stir until just combined.
Add blueberries to the batter and gently fold them in.
Fill muffin liners to ⅔ full and then top each muffin with the streusel mixture. Press the streusel gently into the batter.
Bake for 20 - 25 minutes or until a toothpick inserted comes out clean.
Leave muffins to cool briefly before serving.