Possibly - without chemical spots tests and UV lamp I cannot rule out P. arnoldii BUT this observation does look rather like P. robustum as I know it in New Zealand (see http://naturewatch.org.nz/observations/7670630, http://naturewatch.org.nz/observations/6389802, http://naturewatch.org.nz/observations/6311750, http://naturewatch.org.nz/observations/4766957, http://naturewatch.org.nz/observations/4274491, http://naturewatch.org.nz/observations/3264193, http://naturewatch.org.nz/observations/2867874)

Images show specimen that has been partially wetted (so you can see dry and wet states), and also shown is the thallus underside margin.

Thallus lobes very broad, upper surface smooth (without reticulate cracking), very faintly white-maculate near lobe margins, lobe margins virtually glabrous, (very sparsely ciliate in lobe axils and sinuses); soralia short, on small marginal laciniae, sorediate laciniae convolute, sorediate (white, granular); lower surface with broad, smooth, naked pale brown margin, otherwise finely, copiously rhizinate, rhizines simple, black. Specimen conforms to description offered for P. robustum in the Parmotrema treatment of Galloway 2007).

Galloway, D.J. 2007: Flora of New Zealand. Lichens. Second Edition. Manaaki Whenua Press, Lincoln.

Voucher: P.J. de Lange CK286, F,