The secret initiation ceremony of the Knights Templar - a medieval Christian military order - will be revealed at a public open day in Nelson.

The re-enactment of the historic ceremony will be performed at the Masonic Hall Centre on Saturday.

"This is the only time you're going to see it in New Zealand," said David Marsh, prior of the Nelson chapter of the Knights Templar.

"We've got special permission to do it from the Knights Templar Association here in New Zealand."

The Knights Templar has commonly been referred to as an order of warrior monks.

They protected pilgrims on their journey to Jerusalem during the 12th Century and and fought as knights in the Crusades, a series of military campaigns during the time of Medieval England.

Historically, they wore distinctive white mantles with a red cross and armour.

"The main thing which was different from all the other teutonic orders at the time is that they were sworn to poverty, chastity and obedience," Marsh said.

"They were a unique culmination of both religious people and fighting people."

The Knights Templar is also affiliated with Freemasonry.

Marsh said he came across documents during a recent tour of France that outlined the original initiation ceremony and had them translated in New Zealand.

He said the re-enactment was a "way of making the information publicly available in a way people can understand".

"This is the opportunity they've got to actually be part of it. The ceremony takes place with the people in the room. It's a sort of experience."

Marsh said there were about 30 members of the Knights Templar in Nelson - one of the biggest contingents in New Zealand - and they would be wearing traditional dress during the ceremony.

Historically, a young squire would be associated with the order for several years and would only be accepted if they were deemed worthy.

"This is a very serious step because to join the Knights Templar you had to give them anything you owned and you became totally immersed in them and that was it, there was no way out apart from death."

Marsh denied that the Knights Templar was secretive.

"It's not secrecy it's just there are certain things, like all organisations, we keep confidential. This is a chance for people to get some taste of what it's like."

Copies of the Templar rule book, from circa 1130, would also be handed out on the day.

The re-enactment is at the Masonic Hall Centre in Nile St and starts at 2pm on Saturday.