Yes, it’s definitely a Brown Thornbill. And you're correct that the Chestnut-rumped Thornbill doesn't occur anywhere near Blackheath, being a bird of the more arid country. Also, the Chestnut-rumped doesn’t have the striated throat and breast as this one does. (One of the things I teach my birding groups is that, for identification, markings are generally much more useful than colour - at least when it comes to subtle colour differences. Colour can vary so much depending on the light, feather wear, age/subspecies of the bird, etc…)
Perhaps you meant to consider Chestnut-rumped Heathwren, which is found in the Blue Mountains, but that has a distinct pale eyebrow, pale eye and a cocked tail, among other things.
On 28 May 2020, at 5:33 pm, wrote: