Have they looked at the German Embassy site for New Zealand?
The first link of that page leads to the Covid-19: Informationen für Reisende page that gives details about the present situation.
Have they registred in the ELEFAND as well at the www.rueckholprogramm.de?
the latter is needed so that the Embassy knows who needs help
Hierfür gilt ...
For your proposed routing, Timatic currently says:
Transit - Australia (AU)
Passengers are not allowed to transit or enter Australia.
Passengers departing from New Zealand with a confirmed onward ticket on the same calendar day to return home are allowed to transit through Australia. They must remain airside and must have not been in China (...
The Deutschlandfunk news today talked about the recovery flights and mentioned that smaller Pacific islands were difficult, so they were going to stage the recovery through New Zealand.
At a wild guess, the flight(s) would carry Germans stuck in NZ, too.
I have paid for an IDL, and nobody ever wanted to see it; not in Mexico, not in the US, not in Canada, not in Russia, not in India. If they even know what it is, they decline to accept it, and ask for my "normal driver's license" (which is a German one, btw)
Rental car companies are having hundreds of international customers every day, and they are quite ...