As soon as I so your pic I thought of Kirrily Johnston, an Australian designer ... I checked her website and found these pics - the fabric def looks the same, maybe in the same style as the white?? Retails for AUD$595 ... the have an E-Shop, but unsure if they ship OS
I'm pretty sure it is by Kirrily Johnston. I searched for the collection and the orange fabric is the same and there were some dresses with the same belt as the one Miranda's wearing.