Upwards of 25,000 people - the equivalent of half of Whanganui's population - are expected to descend on Ratana Pa over the next two days, as the church celebrates its centenary.
Politicians including Prime Minister Jacinda Ardern will be front and centre today, while tomorrow is reserved for church celebrations.
The celebrations begin today with a series of welcome pōwhiri - first for Rātana followers, then for iwi, at 1pm for other churches and at 3pm for political parties. Prime Minister Ardern, National leader Simon Bridges and Green co-leader Marama Davidson are all expected to speak.
There's a church service at 6pm, followed by live entertainment on the marae stage.
But the big day for the faithful will be tomorrow, the 100-year anniversary of the moment the Holy Spirit came to T W Rātana.
At 10am there will be an unveiling and at 11am a play performed by Te Kura o Rātana pupils, then a procession to the temple, a huge church service and a hakari (feast), followed by another service and more entertainment.
The celebrations continue on Friday, with programmes for youth, another service and more entertainment.
At the hilltop settlement paddocks are cleared for parking, and cones on SH3 contain vehicles heading for the pā. One huge marquee will house sleepers, with others for a youth programme and for wānanga (learning sessions).
The event is so big that St John will have its command unit and ambulance on site, Whanganui territory manager Jamie Butler said. He's hoping visitors will quickly spot it, and will arrive with their medications and vital details about their conditions and allergies.
Former Labour MP and Rātana apotoro (apostle) Mita Ririnui is to be the spokesman for the church on this occasion.
On November 6 an exhibition about the history of the Rātana movement opened in the archives office at the pā. It's the work of Dr Arahi Hagger, and he said many others were involved.
"I've spent most of my adult life putting it together but it's all been handed down by other people. All I've done is modified it for another generation."
The show will be open day and night until November 11, with wānanga about its content in a marquee nearby.
T W Rātana was the last of the great prophets, Hagger said, and was predicted by earlier Māori prophets. He came with two important documents, the Bible and the Treaty of Waitangi.
The exhibition shows a beautiful history, and it doesn't only belong to Māori, he said.
"[T W Rātana] put the waka on the straight path, with new teachings to unite Maoridom.
We haven't seen the political side yet, but it's coming."