There is a death registered in NSW for a William Thomas JOHNSTON, with parents David and Agnes in 1986. (20678/1986) Not sure if this is "Billie" (William in the death notice).

Will do a bit more looking.
Modified to add:
Death notice for this William:
"JOHNSTON WILLIAM THOMAS - Aug 21 1986, at hospital, late of St Marys, beloved husband of Gloria ... loving brother and brother-in-law of Ruth and David, Meg and Ray, and brother-in-law of the Landow family, aged 66 years."
(William Thomas JOHNSTON married Gloria Hermoine LANDOW in Randwick, Sydney in 1941 (27341/1941).
So the siblings David and Meg fit, and I guess the age is about right?
Do you have when the children of David and Agnes were born? Also, to help look for Agnes, do you have her parents names? In the death notice for David it mentions John McLAREN.
