I notice few colleagues use 'post' almost everywhere. Today one of them said 'lets meet up post noon' and I thought shouldn't it be 'afternoon'? I could be okay with 'post lunch' but somehow 'post noon' felt very awkward.
"post noon" and "afternoon" are not interchangeable because "afternoon" refers to a specific time period during a day while "noon" refers to a single, specific time.
Saying "post noon" means "let us meet sometime after noon." This is very distinct from:
This sentence is incorrect because "afternoon" is a period of time. "Noon" is the very specific time of 12pm. You could say:
Option (7), however, is nonstandard and I recommend avoiding it. It isn't ungrammatical but it still sounds strange.
The constructs "post lunch" and "post noon" are both incorrect. While "post" can act as a noun prefix in some cases, it does not stand alone as a synonym to "after."