There is a lot of info on the NSW BDM,s re whitfields.Lancelot birth registered in SA born NSW Poliac? (Can't find this town but there is a road named similar near Pooncarie)died Menindee NSW in 1899. Menindee is further north of Pooncarie (District of Wentworth) this is where Henry Alexander Whitfield and second wife Clara lived and had children Robert C b1891 and Henrietta A born? but died 1897 Menindee.there may be others.
Mary Ann died 1885 Pooncarie, District of Wentworth, her father listed as Thomas
Henry Alexander Whitfield died 1895 Menindee NSW his parents listed as William and Barbara.
I have not been able to find your Alexanders birth registration,
It seems Henry and Mary Ann went to Glenelg in SA for awhile. Lancelot was born NSW June 1879 Edgar born/died NSW 1880 then they had Mary Victoria Oct 1881 in SA.
Alexander was 21 when he married in 1904 which means he was born 1883ish. If this is his family they may have been travelling back to NSW (by the way Pooncarie is on the Darling River) as Barbara E. born 1884 in NSW.
I cannot confirm all of this, more research needs to be done on the ages of the children I have SA records but notinterstate records other than the online NSW BDMs.
You could try placing a message on the NSW or Vic message board someone may have more detaild info.
Hope this all makes sense. If I can help further let me know. You can email direct. Deb