Crikey, now you're making me think!
I would take it to mean going over and above the normal expectations for the particular type of work.
So, if an office worker, always being punctual and staying late to complete anything unfinished; if a handyman, not doing the bare minimum but making a really good job of it (e.g., I have a guy who's been coming twice a week for the last 13 years, and for 10 of those I'd have said he was a hard worker, while now I find he's often left a job unfinished, or left poop in the stable when he's mucking out etc.)
Also, SAHM's - or SAHD's - who not only look after the kids but have a polished to death house and a garden to die for - you get the idea.