buying L6 gear from the states would mean your warranty is only valid in the US. the Australian L6 service centres and distributer will not cover your warranty, and I've heard they are not helpful even if you're willing to pay for any repairs if you got your gear from overseas. I know this is a pain, but it means if you want to cover yourself, you need to buy your L6 gear from Australia.
now that aside... if you're willing to take the risk because it'll save you so much money (and you are prepared to pay for both ways shipping to get your gear to the US for servicing if it ever needs it) then I think the G90 unit can take 100-240VAC at 50 or 60Hz, which means it's totally globally compatible with any mains power.
look in the G90 receivers manual here: http://l6c.scdn.line6.net/data/l/0a060b4d94724b6334e9bba35/application/pdf/Relay%20G90%20Receiver%20Pilot's%20Guide%20(Rev%20G)%20-%20English.pdf
there's a photo of the rear of the unit saying 90-250V is ok.. in reality I think my G90 receiver says 100-240 on the back of it, so I think you're all ok on that front.
as far as the XD-V70 systems, check out the PDF that's linked here on that page: http://uk.line6.com/support/docs/DOC-2292 and you'll see that the receiver for those uses a DC adapter that's fine for world voltages too. the only thing you may need are travel adapters to get teh right shape plug... or you could buy those DC adapters locally to save on the hassle of plug shape adapters.
Hope this all helps