Agree with Cando, long distances to be travelled to register babies, not surprised if many were missed ... floods, droughts, if men were droving then no one available to mind older silbings if mum went alone ....
Some possible sighting for COOMBS ... info on NSW electoral rolls in 19th century was based on verbal info given to the policemen sent to collect for the rolls. Not all policemen had university qualifications for spelling... and not all could read their own scribble ... Coombs v Coombe ...

Worse still, handwritten lists sent to Sydney for preparation and printing... no internet to check re correct spelling. No funding for morse code telegrams for checking with policeman who may well be on horseback out solving a crime

NSW ER 1878 BALRANALD,
John George COOMBS, residence, of Morna, Darling River, Wentworth police district
NSW ER 1870 BALRANALD
John George COOMBS, residence, Morna, Murray River, Wentworth police district
John COOMBS, freehold, Morna, Murray River, Wentworth p.d.
Thomas COOMBS, residence, Ana-branch, Murray River, Wentworth p.d.
NSW ER 1902 RIVERINA, polling at Moorna
NONE with surname COOMBS or similar.
Richard Frederick ROBERTS, station manager, Moorna station. Other localities at that polling place include Wingillie, Frenchman’s Creek, Wangumma, Bunnwungie, Lake Victoria, Horseshoe. There were 31 electors enrolled for the 1902 roll, 11 including females.
IF that's the 'right' COOMBE family ... perhaps look to the Ana-branch locality
JM