Does he still have the old NZ PR card that didn't expire? It's not exactly honest, but if he has the card, perhaps he can show it at the embassy during the renewal process - or is there a concern the embassy folks might call and check his real status in NZ?

If so, perhaps having the family mail in the app on his behalf is the best approach.

NZ uses a sticker in the passport for an Indefinite Returning Resident's Visa, so technically he still has that plus the PR entry stamp in his passport (they don't use exit stamps, fortunately). I don't know if they would bother checking with NZ Immigration if the IRRV is still valid or not, that's a good question.

Just found out he would have to appear in person, so cannot renew by mail or through family, sigh...this is why I'm happy to keep my US passport, lol!

Sounds like this is his only real option with the lowest risk of the two - if he enters MY via SG without any stamps (not sure if MY immigration will check thoroughly on the causeway), they might ask questions, but he can claim ignorance and just show he is a NZ PM and it's doubtful they will check from there.

It might be more likely the local embassy will ask more questions about his PM status or verify it because they are right there in NZ.

This is a hard one to pick, but at least there are two useful options to consider. He is not alone - plenty of MY and SG citizens hold dual, even triple citizenship and successfully hide it from their home governments. I don't know why these two countries have a stick up their rear about dual nationality - I was told I could have problems as a visitor showing up with my US and Canadian passports on my person if they were discovered, even though I would only select one to enter/exit.

A lot of Asian and African countries with a 20th century history of being colonies/"protectorates" of foreign (mostly European) powers decided -- upon getting independence -- to restrict the ability of their citizens to have dual citizenship; and this was done so as to force the colonists (and/or their descendants) to make a clean(er) break of sorts and reduce the likelihood of foreign interference in domestic political affairs in the post-independence period.

Some European countries are also still pretty restrictive in terms of allowing citizens to have dual-nationality, but that has nothing to do with a history of being colonized by foreign powers ... even as it has much to do with xenophobia too.

Many countries apply automatic loss of citizenship if another citizenship is acquired voluntarily. Hence the need to hide it, which is fairly easy if one is able to play it carefully

A close relative born in Australia but now holds a Canadian passport. On returning to Australia for a funeral was brought in for questioning due to the suspicious short travel on his passport.

He had a job in travel and would visit South American countries to view hotel facilities for a day or two in each country. They started to ask about drug use and what he was doing there. The questioning was quite detailed until they asked if he had held a passport from another country. When he said he had held an Australian passport, the questioning almost stopped immediately and he was let go.

The rights to use multiple passports does not usually come with the wisdom of when/where to use each one.