In the same area we saw the thornbills there was also a Willie Wagtail. It would often perch on the top branches of the small trees to call out. Is it normal for a Willie to have such a mottled belly, I thought they were normally only white in the underside please?
Yes, white underside. Perhaps the base of the white feathers are black and are exposed in this case due to being wet or moulting? Complete speculation here...