Even though I cannot answer your question regarding these two waterfalls absence from Wikipedia. I have searched these names and only one of them is official, McGowans Falls, Tarkine falls is not an official name to any hydrographic feature in Tasmania, it may just be a colloquial name and actually be officially named as something else.
If however it is indeed a definite waterfall feature and has no official name i.e you have found it. You can go through the naming process at nomenclature and assign a name yourself! as long as it is within the naming guidelines. Great conversation starter! Hey did you know I named a waterfall ?