What a nice outfit! You could do it as an offset circle, but I think what they actually did is much simpler and like the bustle skirt that Brenda linked to. It would lend itself to sari-converting (which is also what I think they did).
It looks to me like the outer layer is simply a long, narrow rectangle of material with an extra sewn on border, cut along the lengthwise grain to the desired width and made significantly longer than the waist measurement. The excess, which is probably seamed along the upper edge beyond the waist opening, then falls down at the back into that tail.
If you look at the link Breda posted (
http://carolinejoynson.blogspot.co.uk/2013/07/bustle-skirts.html), you can clearly see how the white threads on the selvage edge form exactly those pleats where the back of the rectangle is longer than the body. I don't think there is a zip, as the upper edge of your skirt has gathers suggestive of elastic, but you could use a zip if you wanted to.
There is also what might be a diagonal seam on the right side that suggests they pieced in an extra triangular panel to give a better fall at the back. If you played around draping the sari, without cutting it, I think you'd find a good way to make it work quite easily.