I'm a Yank married to an Aussie so I run into this same conundrum a lot... You can get most anything in Oz, but as with everywhere, certain items are way more expensive there and make a better gift. Just wanted to throw out a few ideas based on trial & error:

WHAT'S WORKED:

See's candies

Rude novelty hot sauces (preferably with some donkey reference on the label) - I have no idea why, but if you want to make an Aussie laugh, just say something like "If I don't get an op-up I'm gonna open up a can of whoop-@ss on the airline."

Goobers, that disgusting jar of petrochemical based "peanut butter" with the jelly pre-swirrled in. It was received as a brilliant triumph of Yank engineering when it was meant to be a joke..."Crikey you only have to use one knife" (OK, I made the "Crikey" line up.)

Computer stuff, portable USB hard drives etc. Much pricier (especially Apple stuff) there. Just don't get anything that works on AC -- different voltage and plug.

ANYTHING recent "As seen on TV" crap...(Snuggy, Ped Egg, Sham-Wow etc.) They'll hit the Australian market about 6 months after we're sick of em and go for triple the price.

Cosmetics -- good hand cream, bath sets, etc.

If you have a high-end brand store near you (Tiffany's, Bloomingdale's, etc.) an inexpensive gift in that wrapping goes over well. They're quite aware of the US brands through movies / TV.

DEFINITELY AVOID:

Wine! There is so much local pride in their wineries. In a recent wine tasting event with friends to guess the "mystery wine" the first question was: a) Australian, b) European, or c) "Rest of World". As a native Californian, my Napa Valley pride was bruised a little being lumped into rest of world.

Macadamia products. Aussies are as surprised that we think of them as a Hawaiian product not an Australian product. Basically not a good gift on either end of the Pacific.

Cookies, baked goods, etc. I admit that our beloved Oreo doesn't hold a candle to a TimTam.

Clothing unless you remember that the seasons are backwards. Giving someone a sweater for Christmas (when it's probably 100 degrees outside) looks a bit foolish.

Anyway as others have said, make sure you declare ALL food products - including chocolates, hot sauce, etc. They won't take most commercially packaged food away (except honey and meat) and you normally have to run the gauntlet of beagles, German Shepherds, organic X-ray, etc. to make sure you're not an international produce smuggler.