JP National Parents live abroad and buying a property together. What do they need to execute the transaction?

My parents are Japanese nationals living abroad (in New Zealand, to be particular). They've been there for decades.

I (NZ national) live in Japan with my wife and kids and have put an offer (accepted) for a property. Me and my parents are going to be paying for the property together.

The realtor asked for 印鑑 + 実印証明 (Inkan Proof) from me. Which I can register at the city hall no problem.

However, my parents don't have a 住民票 and therefore can't register an Inkan. My realtor still says they can use an Inkan but will still need some verification in the future. I have no idea what this means but I don't think he know either so I'm a bit stuck.

In this case, how do my parents living abroad execute the transaction?

by No-Illustrator-4549