trying to focus on the son who you mentioned, William (John) Gordon who married Evelyn Catherine Gordon. I would like to discover if they had any children and I wasn't able to find any in spite of extensive searches on the census records. It appears that William and Evelyn moved to the Toowoomba district (probably selling the Croxdale farm owned by their parents) when their father died in 1914.
A couple of points in this statement need your attention
Australia has no useful census records.
We do have online electoral rolls which is perhaps what you have been viewing.
There is no evidence that Murdoch was the father of Catherine/Eveline (Evelyn Catherine).
The married couple
EDIT (William and Eveline/Evelyn) were
step siblings, not
half siblings.
Sue
ADDING
Also be mindful that the child Norman Ivan was ex-nuptial.
He was probably registered in the the name GORDON because Catherine McINTYRE ( the mother of Evelyn) was married to Murdoch GORDON and possibly Evelyn used that surname in her every day life to fit with the rest of the family.
In other words, the surname GORDON attached to Norman Ivan is not necessarily because William John GORDON was his father.
The child's registration at death does show William John GORDON as father, but again, it may or may not be the fact.