The Seige of Mafeking was 1899-1900 so William couldn't have arrived in Australia until later when he would have been in his middle to later 40s so a bit old for enlisting in the army.
Australian online immigration records are more C19th.
If the children were sent back to England from South Africa after the seige to be educated then you are really looking at officer class.
There are no Hepworths on the 1901 census in England who appear to have been born in Africa.
At least one of William's sons did serve with the Australian army.
HEPWORTH, GEORGE JELF
Initials: G J
Nationality: Australian
Rank: Private
Regiment/Service: Australian Infantry, A.I.F.
Unit Text: 38th Bn.
Age: 23
Date of Death: between 07/06/1917 and 09/06/1917
Service No: 2333
Additional information: Son of William Jelf Hepworth and Elizabeth Gray Hepworth, of Shepparton, Victoria. Native of Mafeking, South Africa.
Casualty Type: Commonwealth War Dead
Grave/Memorial Reference: Panel 7 - 17 - 23 - 25 - 27 - 29 - 31.
Memorial: YPRES (MENIN GATE) MEMORIAL
Regards
Valda
