looked pretty ordinary from the top, but when I put the mirror in underneath I could see the weird shape and the honeycomb look instead of gills. awesome. not seen this species before.
This is a widespread species and, as you say, looks pretty ordinary from the top. The one you've photographed have dried out a little bit. When fresh those honeycomb cells are elongated in a radial direction. You still get a hint of that in your photos and you can see fresh examples here: https://www.anbg.gov.au/fungi/images-captions/polyporus-arcularius-0044.html and here: https://www.anbg.gov.au/fungi/images-captions/polyporus-artcularius-0127.html (the caption in each case being an older name for this species).
TYVM Heino1! It was memorable for me even though its widespread. My friends ask me why I take a compact with me going bush. I say its for the fungi and they look at me oddly!