Don't forget that you may receive fornightly payments from the government as well which may not be much, but it may cover the expense of nappies etc.

We are not on a huge income but DH and I had 2 children 20 months apart, they are still in nappies, we moved out of Sydney with a lower mortgage (was able to do so as DH's work can take him anywhere) Nappies are the biggest expense, but honestly you do not have to put them in Huggies, there are cheaper alternatives.
When is the right time to have a baby, there are a million people asking the same question. My grandmother who had 6 said there is always room for one more and this would have been in an era where she would have never considered working and my grandfather brought home the $$$. Their income would have been small but they made do. If ever we ponder over when is the right time I always think back to how our older generation dealt with. I thought I was pg again last month and even though this is a time where I'd prefer to not have 3 in nappies sitting looking at our budget we could have dealt with it, and as I said we are not on a huge income.

Do you have family near by, maybe mum or a MIL that would like to look after bubs if you went back to work. When we still lived in Sydney I went back to work one day a week and MIL looked after DD on that day. That lasted about 3 months until we left town. I have since stayed at home and created an on line business which I work from home so I am with the children. I do not want to miss them growing up, I am not prepared to leave my kids in the hands of someone else 4-5 days a week. i totally understand that there are people out there that need to do so, but we decided to leave town get a cheaper mortgage and sacifice a few things to be able to afford to do so. We no longer spend money on ourselves for clothes etc, our budget is so down pact, it's been amazing how well we have done.
I went public with DS and didn't pay anything with his pregnancy or birth. With DD I went private and was out of pocket thousands so look at that too, figure out whether it's public or private you want as that may make a difference.

Goodluck with your decision.