It seems that plagiarism & plagiarist should be spelt with πΌ rather than the etymologically motivated π½, just like the related verb plagiarise/plagiarize and its forms.
OED shows an optional /(Ιͺ)/ in parens for RP; Cambridge has the pronunciation with /-dΚi.Ι.rΙͺ-/ (US /-dΚi.Ι-/), but listed secondary to the one with /-dΚΙr.Ιͺ-/ (US /-dΚΙ.Ιͺ-/). M-W lists it as "also -jΔ-Ι-" giving preference to plain "-jΙ-Λri-". Other sources don't even consider any other possibility than /-dΚΙrΙͺ-/ which seems to be more common pronunciation across the board (idk about Australian, though). All the sources I checked transcribe the stems of the -ism & -ist nouns identically to the related verb.
It seems that plagiarism & plagiarist should be spelt with πΌ rather than the etymologically motivated π½, just like the related verb plagiarise/plagiarize and its forms. OED shows an optional /(Ιͺ)/ in parens for RP; Cambridge has the pronunciation with /-dΚi.Ι.rΙͺ-/ (US /-dΚi.Ι-/), but listed secondary to the one with /-dΚΙr.Ιͺ-/ (US /-dΚΙ.Ιͺ-/). M-W lists it as "also -jΔ-Ι-" giving preference to plain "-jΙ-Λri-". Other sources don't even consider any other possibility than /-dΚΙrΙͺ-/ which seems to be more common pronunciation across the board (idk about Australian, though). All the sources I checked transcribe the stems of the -ism & -ist nouns identically to the related verb.