No worries is definitely agreed to be the MOST Aussie phrase there is. Despite it being easily understood elsewhere, what makes it Australian is the frequency that Australians use it. It's constant.
It often is said instead of "ok". It just means "cool", or "that's fine" or "sweet". And it overlaps a bit with another really common Aussie phrase "too easy".
Eg. In response to:
"I've gotta go"
"Can you pick up some milk on your way home?"
"We don't sell that here"
"I'm going to drop off Kate then come pick you up straight away"
Yeah pretty much any situation that someone wants to reassure someone else even on the smallest level, it can be used.