I had a separate ticket on Qantas MEL-SYD then to Hobart and back to MEL and was traveling with two small bags (I was on a RTW and had a summer Sydney bag and winter sweaters Europe) that totaled the weight limit but was charged for the extra bag on each leg. Plus they make you do your own bag checkin and weight and the computer didn't care about the US laws, bag weight etc - it saw two bags and charged me. Lesson learned - pack one big bag to avoid extra fees.