busy at/with ?

What are you doing?
What are you so busy doing?
What are you busy with?

We don't usually use "at" in a phrase like that.

But you can say

I've been at it all day.
Busy at the workbench
Working at the harvest

I'm a Yank. I can't remember ever saying it, but over here I think we could just as easily say "busy at" if someone looked like they were really involved in doing something, might say "working at" too.