From milk + -o (“diminutive suffix”).
milko (plural milkos)
- (Australia, New Zealand, informal) A milkman or milkwoman
1998 August 15, Seagrass, “Lousy Milk”, in aus.general, Usenet:
Actually, having been denied the right to employment in my chosen occupation, makin' chemicals, and being now a milko, i can give you the inside information on this story.
1998 December 17, David Bennetts, “Neutral Bay tram line”, in aus.rail, Usenet:
In the resulting commotion dogs barked and the milko's horse shied, leaving broken bottles and spilt milk all over Hayes St.
2001 March 11, Brash, “Multiculturalism leaves no place for Australian culture”, in canb.general, Usenet:
(As a child, I used to love hearing the "clip clop clip clop" of the milko′s horse as he arrived in our street in the morning).