When Martha WALKER dies, 1878, there is someone around with enough interest in her to place a death notice in the newspaper.
For all that Martha is said to be well known in Rockhampton, I do not see anything in the newspapers for her before or after 1878.
And there is someone around to put a headstone on her burial plot? The headstone is large.....so intended for burials later that 1878 suggesting....other WALKER family?
Martha is the first?...and only?....burial in this plot?
Apart from inscriptions on the headstones, do you have access to cemetery records?