Well, I believe we invented it in it's modern incarnation, so yeah, it's pretty big.
It's not an "offical" government holiday or anything (we don't get days off from school/work/etc.) but most people do celebrate it in one way or another.
I have friends in Switzerland and other EU countries that say it's finally catching on over there. The only people I know who aren't involved are heavily religious people that think it's somehow demon worship or whatever - but to us it's really just a fun day to play jokes, dress up in silly/scary costumes, and eat a lot of candy. I know more adults that have Halloween parties than kids!
I hope you have a great trip! Oh, and please pass the vegimite.
N.E.D., who has known enough people from OZ to know that Koala's aren't bears