That's rough to decide when I'm looking at an egg, let alone over the internet. 12 hours is not all that long for a chicken egg, not sure about pukeko. Any time you have to help there is something wrong, that means it might not survive. But you do the best you can.
If you help too early, the risk is that it has not done all it needs to do before actual hatch. That could be absorbing the yolk or drying up the blood vessels. It has learned to breathe in air by now.
This is not my strength either but I'd agree to remove a little more shell only, not membrane. If you see any blood stop immediately. Go slowly, but as long as you don't see blood keep going until you can see how dry that membrane is. I'd probably keep going as long as I did not see anything to stop me. and I would be very nervous.
Have the brooder ready. That chick needs to be in a warm place when it comes out. I would not worry if bits of the membrane are stuck to it as long as it can move. If the down looks matted down, don't worry as long as it can move. I had a chick I helped have matted down for a week before it cleaned itself up.
I'll tag
@casportpony She probably knows this stuff better than I do and will at least enjoy learning about this species.
