I would be tempted to find out the average HB cost and budget for it. Be upfront - maybe state "I understand HB are roughly $xxx. Unfortunately we had budgeted against the FBC and are able to pay yy% at tt(time) with the balance after birth". (although the baby bonus is in fortnightly payments?)

Might also be worth looking at your bills and pre-paying what you can so you can get a bit of a lump sum ready. good luck Teni