It seems to be a fairly common complaint that Aussie gamers get bad ping times when trying to play with people overseas (particularly in the U.S., which I guess is because that's where most of the Xbox Live users live). A lot of the time, the frustration is directed towards to Microsoft (i.e. "MICROSOFT dont care about us auzzies", "MS don't give a *** about us and will do nothing to facilitate our connection to international gamers", etc) but the thing is, it's not really Microsoft's fault. You can blame simple phsyics for it.
See, as far as scientists understand, information cannot travel faster than the speed of light. Now, using a bit of mathematics and knowing that light travels at 300,000,000 m/s and that Sydney -> L.A. is ~20,000 km you can work out that a beam of light would take around 66ms to get from here to L.A. Ping times, of course, measure the round-trip time (there and back again) which gives a physical minimum ping time of around 133ms.
Given that an internet connection is not a simple beam of light - it has to travel through a number of servers on the way (e.g. a couple of servers at your ISP, the first tier ISP, the international link, etc) I'd say 300ms is pretty good.
The real problem, I guess, is that Live doesn't always make it easy to find local games. There's also been a couple of bugs in games where people in the US get preference for hosting and stuff like that. I guess that's because most game developers are in the U.S. and while the entire population of Australia is 22 million, in the U.S. you've got individual cities with a population of 22 million. That's a pretty big market difference!