There are many different accents, In fact accents don't only differ between countries, but within the country too. For example, somebody that lives in the top region of Australia usually sound different to people along the bottom (which I find has more of an English accent) and again over in the west have a subtle difference as well. I'm not sure if the same applies to America however I dare say it will be the same story. There are sometimes even different accents between class, As those who are upper class may speak with a different accent to others (sometimes intentionally). The short answer is Yes, The differences between an American accent and Australian accent are extremely noticeable. On a side note, just as an observation from an Australian, If you ever see an "Australian accent" on TV, I find that most of the time they get the accent wrong and we don't actually sound like that. (most of us anyway)

That is quite a big difference. In a sense, you might doubt if your ears have problem and think if they are speaking English especially you never met Aussie speaker with strong accent. By the way, you had better not start with English learning with Aussie....where possible.

Yes, of course! they ARE different: Australian sounds (at least to my ear) softer, and vowels are not as broad as in American. There are obviously differences in vocabulary, but let's just focus on pronunciation and the 'music' of it. Since most people are familiar with American accent(s), here is a video with someone speaking with Australian accent: https://www.youtube.com/watch?v=xGGC2YLlkGI