Hi Paul, so the way I figured out what species of Thornbill was down to a few features. Brown thornbill (Acanthiza pusilla) have red eyes, which in the first photo you can see isn't the case for this one. Both photos show that yellow underside coupled with the ground foraging that you observed narrowed it down to two species, Yellow-rumped thornbill (Acanthiza chrysorrhoa) and the Buff-rumped thornbill (Acanthiza reguloides). The difference between these two is that the Yellow-rumped thornbill have a black and white spotted area on the head just above the beak which the bird in your photo seems to be missing. Hope that helps