Leslie possibly added a couple of few years to his age so he could enlist without parents' consent (both parents were deceased) and maybe his sister or guardian wasn't keen for him to enlist ....?
That birthdate tallies with NSW death age of 66 for Leslie
Ros
Which birth date tallies?
The one on his father's family details form, or the one given on his enlistment index?
In 1980, as mentioned by OP, one Leslie Edward CASEY was living at Mt Isa.
With him a woman who had been registered at the same address as Leslie since about 1949, and three others of the same surname.
Address 61 Pamela Street, Mt Isa.
The man was a timber worker on many of his enrolment entries.
Sue