There is a Mr Cecil ARMSTRONG arriving from London per the Orama in April 1928, for Adelaide (no others for Adelaide surnamed Armstrong on that voyage.
http://soda.naa.gov.au/record/1000657/1On the other hand, there is a submitted tree, perhaps attributed to our OP, with Kenneth Vincent ARMSTRONG's death 17 December 2003, Meadowbrook with parents as Cesar Roy ARMSTRONG, 1896-1968... with Cesar Roy's birth as Bombala, 1896, son of William Armstrong, 1874-1938 and Sophia Whittaker 1874-1917. Mary as Cesar's wife is shown as Mary O'BRIEN, 1904-1997.
William and Sophia ARMSTRONG's children:
George William Clarence 1894-1918
Cesar Roy 1896-1968
Julius Serle 1899
Alfred E 1902-1903
Florence M 1910 -2000
Ronald Hamilton 1911-
Right ... so earliest ARMSTRONG on this submitted tree who died in NSW seems to be James ARMSTRONG 1810 (Glasgow) - 21 March 1867, Greigs Flat NSW who married Elizabeth JAMIESON 1816-1899. Their daughter Mary Ann SLATTERY (no, I cannot see how this lass became SLATTERY, it seems to have been her birth surname) 1847-1890 was born 1847 at Cunningham Point near Bombala.
Not sure how any of that helps, I am always so wary of submitted trees...
JM