Thanks both, it's good to know the hospital will have the forms.

Yes, DH runs a small business for his work, has the ABN etc. I get confused by all the details of how all that works, but he knows the mechanics by now, I think (only been doing this for a few months). I think we can estimate his income nearly enough not to go much over or under, in which case, in my opinion, the money is better off sitting in our mortgage for the duration and we might have to pay some of it back.

Do the tax office or centrelink charge interest or fines if we are overpaid by accident, we notify them ASAP and we pay it back immediately?

DH has actually earned a lot more in the early part of the year than we expect him to in the latter part of the year (partly because he'll be working less for a couple of months when baby comes, partly because some people paid him a large amount of money that he earned last year really late, so it has come in this year). Will they want to estimate our income for the coming tax year based on the recent few months? In which case they are likely to overestimate anyway.