Hi there.
Im in the exact situation you're talking about.

Its difficult, DH earns enough money to cover our mortgage, just, but nothing else, Im ean nothing - food, bills, fuel etc

We tried to save up $ to pay the mortgage so we could live off his wage and the baby bonus, family tax benefit.

However with one thing or another this pretty much eroded the savings and we are now at the point where Im trying to time my tax return so I get back $ in time for mortgage payments.

Talk to centrelink about what you are entitled to, family tax benefit etc, and anything else.
Start trying to put away what you can now, which you are still working.

It's very daunting. Im looking at, best case scenario, going back to work in November when DS2 will be 6 months old, when I really wanted to be off for a full 12 months.

You do what you can. You can still feed with breastmilk even if you arent breastfeeding, check out the ABA booklet on expressign and returning to work.

If you have a mortgage you may be able to get repayments put on hold, some bank so this.