I'm doing DCA-JFK-LHR-BCN on AA/BA at the end of the month and will let you know how it goes.
As Dave Noble said, check the bags through to Sydney. If there are any problems, there are enough flights to get you your bag in a timely manner.
If you are still concerned, pack 1-2 days gear in a carry-on and place the rest in checked baggage. If travelling with a companion, include some of theirs so you both aren't stuck waiting for baggage or having to purchase new clothes.