I would definetly talk to Centrelink and see what they say.
My reaction was anger at him as it takes two to tango, if HE didn't want children he should have made sure it didn't happen. You did the right thing and took precaution, what did he do except enjoy his part.
I will be thinking of you as you deal with this and know you will make the right decision, for you, when you decide what to do.

I would tell your Mum.
If this was my DD I would be upset but would support her. Yes, it would be a shock but she is still my DD