In 1923 the weather was grey and wet for the opening, and the forecast is looking similar for tomorrow.
Organiser Diane Gordon-Burns said they had capped registrations at 450, and those people would be able to catch a bus to the tunnel portal. Along the way they will enjoy performances from actors representing the people of Otira 100 years ago.
While it was too late to book a ticket, she has had late queries and encourages people to come anyway.
The Otira heritage book, launched in time for the centennial, has sold so well they have printed a further 300 copies.
Guests on Friday include Kiwi Rail, the mayors of Westland and Selwyn, Ngāi Tahu kaumatua Sir Tipene O'Regan along with other iwi representatives, and rail heritage people.
Many people with family links to early Otira had also signed up.
Dr Gordon-Burns said she hoped the event would "put Otira on the map".
"We are very humbled by how many people have rung, e-mailed, written and texted, who had ancestors here. It is well known by so many people. The event will bring people together, and acknowledge those (who worked on the tunnel)."
On Saturday, the festivities move to Arthur's Pass, with a photo exhibition at the Community Centre, the opening of an updated Historic Walk including a guided walk and viewing of historic tunnellers' cottages. Bealey Hotel will have a tunnellers shindig, to which bookings are essential.
Official programme
• 10am: Registrations open in marquee outside hall — collect your ticket; Kokatahi Band playing on station platform.
• 10.30am First bus leaves for tunnel viewing area. Buses will leave when full.
• 11.15am: Tranz Alpine arrives — welcome on platform.
• 11.45: Invited guests (speech makers and partners) leave for tunnel viewing area.
• 2pm: Speeches in hall.
• 2.45pm: To station platform to view plaque unveiling by Mark Heissenbuttel, formerly South Island Kiwi Rail manager now heading the introduction of new Kiwi Rail locomotives.
•3.20pm: Cake cut by Lynnette Massey O'Connor, great-granddaughter of former prime minister William Massey, who was present in Ōtira for the tunnel opening in 1923.
• 3.45pm: Karakia whakamutunga (closing prayers). To platform to farewell guests.
