bless you, I also fell preggers on a 457 whilst here and think I went through similar stresses you are having now. Dont fear though, as already stated you can go all out on medicare. You will have to pay for the scans though, although these arent too much but everything else is free.
The only disadvantage is you cant pick/choose your gyno etc.. however having experienced NHS system with first bub this was much much better, I was checked every 4 weeks up until due date with fabulous care.
When you land, go to your nearest medicare office straight away with your visa paperwork. They will then process you and you will get a recripocal card, this will be the number you use for the hospital.
It would also be worth touching base with your nearest hospital before you leave.
good luck.