Possibly - tentative determination by Dr Robert Luecking "S. kelica" - I will need to check spores etc.

Thallus silivery white but without obvious prothallus. Thallus covered in sorediate / pruinose orange-yellow pustules (pycnidia?). These structures K+ violet rapidly changed to black.

Common on pohutukawa (Metrosideros excelsa) in extremely dry situations - associated with Calicium tricolor and Chrysothrix xanthina.

Voucher: P.J. de Lange 14394 & M. James, UNITEC 10704

Stirtoniella kelica - least ways as we understand it in New Zealand looks like this - https://inaturalist.nz/observations/13508617, https://inaturalist.nz/observations/19097349