You need to call Qantas (QF) and have them rebook you on an earlier AS flight from Seattle. Another alternative: you can ask to be rebooked to SFO and then on to SYD and MEL.

In any case QF is responsible for rebooking you. 40 mins will probably not be enough time.

If your JNU-SEA tix are impacted by what they (QF) offers, get all 3 airlines on the phone and try to work it out.

Let us know how it goes.