Bec, I've come late into the discussion, but I agree with Lucy (I'm not from a HR perspective, I'm from the union delegate perspective!), give them the bare essentials of information and if you get pressed beyond this just say that you don't have to discuss it.
Employment supplements of weekend papers are full of maternity leave positions, so if your boss claims it's too hard, then you're the least of his worries.
Regardless of whether you want to return or not, you stick to your entitlements. If you find something else near the end of your leave, give the appropriate leave (outlined in a contract, award or agreement) and take off. Your boss has asked for it, quite frankly. And he DOESN'T have to pay casual wages, unless he advertises for a casual, instead of a fixed term full timer.
Put it this way, if you roll over and let him bully you into resigning before you take your leave, it makes it so much easier for him to do this to someone else - and it's no skin off your nose, really, to just say you'll come back (even if you don't). Your allegiance should not lie with someone who is trying to swindle you out of entitlements that women have worked bloody hard to receive in the last few decades.
As for your conscience, let it look after you, especially as it is all too clear that your boss is looking after himself and his own interests. Don't help him out, regardless of your inkling that you won't go back.
My manager asked if I would be coming back (in her "I really, really care... but probably not about you" kind of way) and I told her I would. How does this even matter when I'm casual?? Ugh! The Fed Govt was trying to say that most employers do the right thing... how come I only know of the ones who don't?
Sounds like you're on the ball and getting your confidence back about the scenario and what to say to him