The appearance is consistent with Xanthoparmelia - but there is a chance it could belong to another genus in the same family, Parmeliaceae. If you want to bet on it, go for Xanthoparmelia, simply because it is by far the largest genus in the family. Species of Xanthoparmelia are mostly found on rock, bitumen, soil - rarely on wood. You can read more about the genus in Australia here: http://www.cpbr.gov.au/lichen/xanthoparmelia.html.
Volume 55 of the Flora of Australia series deals with the Parmeliaceae and there you find the comment that Flavoparmelia rutidota is "probably the commonest lichen found on dead wood in Australia". In the ACT you find it in nature reserves and in suburbia. Very often you do see large coverings as shown in the two photos of the tree trunk. Strictly speaking I should say that I'm 99.9% sure this is rutidota - since other species of Flavoparmelia do occur in the ACT .
Hi Heino, Many Thanks for your comments & references. To the best of my recollection (I would have to Field verify this from the specimen location) the lichen/fungus was growing on LIVING trees (not dead wood) in this area on Multiple tree trunks.Not sure if this is significant ?
Hi Heino and Max
I found similar llooking Lichen on the bark of living Callitris endlicheri (there is a small grove of trees on the west slope); see this photograph:
https://www.flickr.com/photos/61627737@N03/5960329121/in/pool-1745488@N23/
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