Combine parsnips, carrots, celeriac, turnips, kumara, and butternut pumpkin in a large roasting pan. Drizzle with olive oil, and season with 1 teaspoon of sea salt and 1/2 teaspoon of pepper. Toss vegetables to evenly distribute seasonings.
Roast in the preheated oven until the vegetables are easily pierced with a fork, 30 to 45 minutes, stirring every 15 minutes.
Meanwhile, melt the butter in a large pot saucepan over medium heat. Stir in the celery and onion; cook and stir until the onion has softened and turned translucent, about 5 minutes. Pour in the vegetable stock and bring to a simmer, uncovered. Stir in the roasted vegetables and continue simmering for 10 minutes. Puree the soup using an immersion blender. Stir in the extra light cream, and season with salt and pepper, if necessary. If the soup becomes too thick, add more vegetable stock.
Kumara is known in Australia as a Sweet Potato.
Celeriac is also known as celery root. If you can not find this, you and add extra celery for the same flavour.