OK - I just checked Wiliam Topping's war service record and, when he enlisted in March 1916, he originally gave a next of kin, details of which are hard to read, but that was crossed out and "Mrs Mary Margaret Topping, Gordonvale" written over it in pencil. So it seems that, by 1916, he and Mary may have had some sort of reconciliation and she was living at Gordonvale which is a town just south of Cairns (actually almost close to it now as development has spread out). It seems William was granted a pension in 1918, with his address as Gordonvale so it does look like he and Mary reconciled.
The file also contains a letter from Townsville Legacy, Lower Burdekin and Home Hill Group (this is south of Townsville) dated March 1962 regarding an application by his widow (he died 12th March 1962) for his details. There is also a handwritten letter dfrom William himself in 1960 giving the address as Cox Street, Ayr so it looks like Mary was living in the Burdekin district (Ayr/Home Hill) in 1962.
However I can't find either of them buried in th Burdekin or Cairns cemeteries so maybe they were interred in Townsville.