I think what might have given them that idea is the letter that the Family Assistance office sends out to those who are close to the line income wise. We got it too and it says that you have to prove your income for the six months following the birth ie payslips which naturally means you can't prove it until after the six months have passed.
I'm actually going to contact FAO tomorrow regarding this so will let you know the outcome.
I spoke to them nd they are happy to accept a letter from your employer stating what you will both be earning to make their decision.