* Slightly more open than the "oo" in "moon": It's closer to the "oo" in "boot" or "pool" in some American dialects.
* Less rounded than the "oo" in "moon": Imagine the "oo" in "moon" and then slightly flatten your lips.
* Like the "u" in "cup" but longer and more open: This is probably the easiest way to describe it for someone unfamiliar with the sound.
It's important to note that even within Australia, there are regional variations in the pronunciation of "oo." Some regions might pronounce it closer to the "oo" in "moon," while others might be even more open.
Ultimately, the best way to learn the sound is to listen to Australian speakers. You can find many examples of Australian English on YouTube or other online platforms. Pay attention to how they pronounce words like "look," "book," and "good."
I hope this helps!