“This Arvo” Simply Means This Afternoon

When an Australian says, “I’ll send it through this arvo,” they mean, “I’ll send it through this afternoon.” Arvo is an informal shortening of afternoon. The Australian National Dictionary Centre records it from the 1920s and identifies it as part of a familiar Australian pattern that adds -o to shortened words.

Say this arvo, not in this arvo; standard English also normally uses this afternoon without in. For example: “Are you free this arvo?” “After three, yes.” The expression suits conversation and relaxed messages between friends or colleagues. In formal email, or with someone unfamiliar with Australian usage, this afternoon is clearer.

https://slll.cass.anu.edu.au/centres/andc/meanings-origins/a

https://dictionary.cambridge.org/dictionary/english/arvo