I think but don't quote me on this for an international flight itis either 14 or 21 days......it also can vary between airlines with their rulings on stuff like that.

I'm with LS hoping and praying for you that DH can make it back in time and se his beautiful child born!

ETA: Just found this on the Qantas site:
"If an infant is less than 7 days old or not in good health, then medical clearance is required for travel."
So the minimum would seem to be 7 days...don't forget you will need time to arrange the birth certificate to be processed amd stamped before you can apply for the passort and then you have to wait for that and apply for a visa for Britain as well so keep all that in mind when planning as it could take more time than you think.
Cheers!

Because I'm an Australian national and baby's dad is a British national, she will have dual citizenship. I don't know what's the deal with passports...if she'd have a British or Aussie one or both.

I would have to get everything expedited. Obviously, fiance and I don't want to be apart for any longer than we absolutely have to - especially not with our baby!