You have two different things to worry about. Video Format, and region code.
The US is one of the contries that uses NTSC (NTSC DVDs are 720x480). Australia is PAL (PAL DVDs are 720x512 or 525 not sure but it's higher...)
Region Code: Thanks to the pin heads in Hollywood most DVDs are region coded, the US is Region 1, Australia is Region 4, so a US disk will not play on an Aus player... but...
They can be hacked
What you want is a "Region Free/Multiregion" player that is both PAL and NTSC compatible. They are available in the US but harder to find since only fans of Forein film want one, it should be easy to find one in AUS (or any other non Region 1 area) since all the big Hollywood films come out first and cheapest on R1 disks.
When you get to Aus get a TV that supports both NTSC and PAL (all but the cheapest do).
That will let you play DVDs from anywhere in the world.
Tom.