Why would you want to remove a shaft that is plugging a hole in your boat only to replace it with something that is probably not as effective, just before crossing one of the rougher seas on the planet?
If there is a problem with leakage then replace the packing and secure both ends of the shaft. Putting an anode on the shaft and checking that the coupling is secure is essential, but doing more would be advisable.
As for securing the outboard to the ferro transom I would expect that if the ferro itself and the fitting and
backing plates were less than perfect the movement of the boat would enlarge the holes, leading to possible damage that could compromise the
hull.
I'd strongly recommend removing the outboard and securing it below
deck if this can be done safely.
You may also care to ensure that all
fuel containers are secured so they cannot come loose and do not leak.
As the boat may have little
customs value can you not arrange with
Customs to leave the boat in
New Zealand and fly to
Australia, returning with sufficient cash to do the job properly?