There are different degrees of gratitude you're expressing with a "thank you":

- Thanking for an ordinary action, i.e. after buying something at a shop: no answer.

- Thanking for something that didn't cost me very much, i.e. lending £20 to a friend: I usually answer with "no worries (mate)".

- Thanking for something that cost me in some way (time, money, effort), i.e. helping pushing a stranger's car on the street: "You're welcome".

I've heard Australians say "no worries" (often with a trailing "mate") a lot. I always thought it originated there.

As an Australian I can say that "no worries" is very common here (ocker). It has a bit of an ambiguous meaning as it implies that the person saying thank you didn't really need to, but at the same time it is a means of acknowledging that you recognise and appreciate the thank you.