Preheat oven to 180 degrees C. Line a 23cm pie dish with the pie shell, and place the pie dish onto a baking tray.
Place zucchini into a food processor, and process until pureed. Transfer the puree to a bowl and whisk with eggs, white sugar, melted butter, flour and the vanilla essence. Pour the mixture into the pie shell.
To make crumble: Place flour, brown sugar and butter into a bowl. Cut the mixture with a pastry cutter until the topping resembles coarse crumbs. Sprinkle the topping evenly over the pie filling.
Bake until the filling is set and the topping is lightly browned, about 45 minutes. Allow to cool for at least 30 minutes before serving.