I've got the right death and the right siblings:
The Sydney Morning Herald - Saturday 20 July 1963 p.72
DEATHS
JOHNSTON, David Leslie.-July 18, 1963, of 52 Glebe Street, Glebe, dearly loved husband of Vera, loved father of Veronica, David and Leslie and fond brother of Eleanor and William, aged 56 years.
We might have to rely on tracing the children.