I can't be sure from a photograph, but I strongly suspect that this is a corticioid fungus. Mostly you find such fungi on the protected underside of wood that's lying on the ground, but a few (in more than one genus) are found in the exposed habitat of the bark of trees. They can be spread over a considerable distance, in numerous small patches rather than as one continuous sheet. White is the usual colour for such fungi. Of course there are also white crustose lichens but, in Canberra , when I see something that looks like what you've photographed (especially if it's growing on the trunk or on the underside of a branch), I think it's more likely to be a corticioid fungus.