Pleiades – you got a link or extract from an ICAO doc that supports this ?
ICAO don’t actually set Laws – they set “Standards” (or Guidelines) – it’s the down to Country to write their laws IAW these Standards.
So it’s possible the ICAO have said something along the lines of “electrical power failure = pilots decision” and the NZ Govt have said “where possible the flight should continue to destination”, conversely another government may have said “unless 100% certain that fault will not reoccur divert to nearest airport” – both would be IAW ICAO standard – but both would result in different action.
One thing that may need to be taken into account is availability of a “suitable” airfield – along this guys route they would have been everywhere – in NZ they might not be as numerous.