We also have this in certain corners of the U.S.:

A: "'sup [man]?" B: "'sup?"

(It's also stated only has like a half-question or less—the usual question lilt is deemphasized.)

I've always considered the standard reply to be "yeah nah, not too bad, yourself?". I think that it's possibly more Kiwi than Australian though.

usually we'd do a yeah nah, as an optimized "I hear what you are saying, but that's not true... < inform the person of the truth >"

as in "You're looking well", "yeah, nah, been a bit crook lately"

I love how often phrases start with "yeah..."

As well, the South African “howzit”, which from my observation isn’t even actually a question but just invites the other person to say what they wanted to say.

In the south of England we have "alright?". The only valid answer is "alright?.

"Alright" I think is used all over England, maybe even the UK, it's not specific to the south :-)

Yeah I find that annoying. It means nothing and is just a waste of everyone's time, much like:

"How are you?" "Not bad, yourself?" "Good, so I have this problem..."

"Hey, how was your weekend?" "Not bad actually, you?" "Yeah not bad"

I mean, you clearly have no interest in other person's life, so why have stupid, dull, pointless greetings like that?

In Germany, people say "Na?" or "Na du?" to which the other person usually responds the same.

It's not easy to translate but I think it's quite similar to the greetings mentioned above. Literally it's something like: Well? You? (and not 'well' in the sense of goodness, but rather questioning)

It's an interesting greeting and as a foreigner seemed very odd at first.

Which part/s of Germany did you hear that in?

Several areas, but mostly Northern Germany. It's also quite common in movies/TVs, so I think it's quite widespread.