Well, you missed off the connected question, why is the spelling of the Australian Labor Party using the 'or' form. Wikipedia is inconclusive on the topic. From what I read on it while researching the ALP it was either a homage to the US labor movement or it was a common spelling at the time.
Wikipedia has a note on Australian English in the late 19th until the mid 20th century.
I think it is probably due to editorial decisions at newspapers from that time because the same usage does frequently occur in Australian books from the same period. If anyone has a copy of any Australian style guides or editorial guidelines from that time I'm sure we could answer this question.
I have some dictionaries and books on writing from late 19th century so I'll update this later if I find anything.
Don't dismiss the inability of editors in small town and regional newspapers as a possibility either. Australia has a long history of incompetence in that department.
Update 1: I found this article which references an article from before Australian Federation (pre-1901) that blames the problem on the availability of American dictionaries. The article cites papers that I can't find online but they are probably hiding in some upstairs section of a university library.