I think the bunyip was generally a water dwelling cryptid while the yowie was the classic big mountain man, though the names as you say do change a bit with location and over time. There is a very good book called 'The Yowie' which documents hundreds of sighting in Oz. The web site for the book is here http://www.yowiefile.com/The_Yowie_File/Home.htmlsame myth bunyip/yowie terminology is dependant on geographic origin of the user.
think of it like pots and middies.
After reading the book I was convinced that the people where seeing something, but what?