https://www.newshub.co.nz/home/politics/2022/09/queen-elizabeth-s-death-why-nz-s-public-holiday-isn-t-on-same-day-as-uk-funeral.html
にあります。

"We thought it was important that our Governor-General was back. She doesn't arrive back until Friday next week," she said. "Flights are pretty difficult at the moment.

"Only a couple of days later the Prime Minister arrives back, so it seemed right to do it on a Monday and so that's it. It's just basically being practical with regards to the leadership that we think needs to be there."

総督は今週の金曜日（この記事が出たのは先週の金曜日なのでここのnext weekは今週の金曜日）まで帰国せず、首相はその二日後に帰国するので、国のリーダーがいないところで実施するよりはということで月曜日になったようです。