I hear (and use) it frequently, but never to say, "you're welcome". As a matter of fact, used that way it would sound odd to me. To me it means what has happened isn't a problem or at least not a big one. "You don't have salmon? No worries, I'll take halibut." Alaskan here.
Certainly in Australia, no worries can be used to say you’re welcome, but that does not cover it's actual meaning. I've read a lot of articles which say it is the same as no problem but it's not that either.
In all the usages I've heard, it basically says I understand or I appreciate what you are saying, but I don't want to respond with a formal phrase like you're welcome or all is forgiven or I am behind your revolutionary ideas and will follow you into the battle and die by your side.
BTW in Australia instead of no problem we would say she'll be apples or she'll be right.
I didn't know it was Australian... I live in New England and I hear some people say it, although it's used more like how "That's okay" or "It's fine" would be used, instead of as a "You're welcome" type of thing. I don't hear a lot of people say it though, but I know my voice teacher does, but she's from the New England Area, too, I believe.