We are on one income & I have to say that it's amazing what you can do without when you have to! We have had some very tight months but always manage to make it to the end of the month. It did play on my mind when I started maternity leave though so I understand your concerns.
As others have said, you will probably be entitled to some payments from Centrelink when bubs is born.
There are some great threads on here on budgeting & meal planning that you'll probably find very useful.
If finances allow, stock up on nappies & pantry items now. Also cook up some meals & freeze them.
Or use cloth nappies. Although there is an initial outlay you'll appreciate not running out of disposables at inconvinient times!
These are things that have helped us. We also don't go out for dinner or movies anymore, although this is more to do with having had a baby it does save some money!
If there is a toy library near you join when bubs is old enough. For a small fee you'll be able to get different toys on loan instead of buying them
I always take advantage of the sales to layby DS' clothes for the next season. As in during summer I'll layby his winter wardrobe, can be tricky to estimate sizes but well worth it!