We moved abroad earlier this year. I moved "abroad" earlier in my life, but it was only from Canada to the US, so not nearly as much logistics with that move... Are they moving temporarily, or permanently? Are they responsible to move their own things, or is it a corporate move where the company is paying for shipping their stuff over?
No one got us any going away gifts, and I'm honestly not sure what would have been all that useful, to be honest. But, a few service-type things might make for an awesome coupon book, if you'd be willing to do something like that... It would take more time than money on your part, but honestly, that's way more helpful than a gift they'll have to figure out how to lug along (unless it's a corporate move, and you can get the gift to them prior to the packing date by the moving company). _Don't_ get them something bigger than a card or a pair of socks if you are giving it to them after their household is already packed up - suitcases and airline weight limits are finite, and it's tough to fit in everything you want/need to bring with you (even if they are shipping things, it can take ages to get across the world unless they are paying a substantial premium).
Anyways, service type coupon book items that we would have found incredibly helpful: *Ride to the airport before leaving *Homemade meal in the last bit of time before heading out (you could take them out, but honestly, you eat out a _lot_ in the midst of a move, that much more so during an international move... homemade food is awesome) *Pickup/Dropoff at the airport for a future visit home *US purchase and shipping offer - you'd buy something in the US that they can't get where they're at and ship it to them... Yes, you can ask people to help with this type of thing, but knowing someone is willing to do it ahead of time is really nice; word this how you want - there have been a couple things we have had trouble finding here, and one item specifically that I needed a friend in the US to re-ship to us here because we couldn't get it mailed directly from the company *(if you are feeling _really_ generous, and they are shipping their car) the loan of one of your vehicles for a few days/a week after they have dropped theirs off at the point of shipping (the logistics of figuring out how/when/what to do after dropping off the car was a serious headache for us)
Otherwise, some guidebooks, maps, etc is a good idea, if they don't already have them. Maybe a small photo album with shots of them/you/other buddies in it. Think small memento...
People gave us some "moving" gifts this past Christmas because we were supposed to move overseas in Feb. Most of them would have been left in storage or given away before we left. We didn't want to do sea cargo and wait months for our stuff so we had a 1500 lb air freight allowance. Which isn't that big when you are moving households.
I like the idea of Skype credits. You could also do an IOU of sorts. Tell them that you'd like to send them some stuff from the US once they're in Australia, settled and realize what they cannot get there.