@RRizzo *******
the most important thing is that you can skip all the visa hassle for your baby. You give birth, the hospital gets Thai birth certificate sorted out for you (takes 3-4 days), as far as Thailand is concerned, you're done. There's absolutely no need to go back to Australia / do DTV or anything.
Just get your Australian citizenship application submitted in Oz embassy in Bangkok at some point (you can chill for a few years before doing that) and then get a passport for your baby.
Eventually, when you leave Thailand with your child, he/she will get a stamp in the passport saying "Born in Thailand" explaining missing entry stamp. That's it - it's not counted as overstay or anything.