Hmmm. Tricky. i guess it depends on alot of things. As you said you don't think you want to go back there after you have bubs and i am guessing all that travelling will become tiring and more uncomfortable the further you are through the pg. If you like the sound of the new job maybe take that. They are willing to give you time off for your appointments and the contract will be up around the same time you had planned on leaving your current job. Once you have had bubs you may be able to do a bit of casual work for them when you feel ready before you move back to perth.
Anyways that is my thoughts on it but all i can say is only you and DH can make the decision.