The seeming lack of info tying in with the details that we know, suggests all may not be as straightforward as we think.
Possibly the loss of documents on the docks in Melbourne wasn't accidental and there is some mystery to unravel.
Ancestry has a record of a Kathleen Fitzgerald arriving in Halifax, Nova Scotia, however I can't access the record to see if it could fit. If it could, the possibility of her marriage to someone of a different name and subsequent birth of a male child at Three Rivers in 1898 is a slim possibility. Does that seem feasible?
gv