A road cone has been perched on top of the flagpole on Nelson's Christ Church Cathedral.

The mystery of who put it there and how they got up there had lunchtime visitors to the Church Steps stumped on Monday.

"Holy moly," said one pupil who was there for lunch. "How'd it get up there?"

The cathedral tower has a ladder accessible from inside, but it's believed the person, or people, responsible scaled the outside of the building and then climbed the flagpole with the cone.

The tower is 35 metres high.

Christ Church Cathedral dean Reverend Nick Kirk said he "wouldn't have a clue" how the cone got on the flagpole, or who put it there.

"People do sometimes climb up on the roof and things and they've obviously managed to get up there somehow."

He believed the cone appeared overnight Sunday.

"I think it's just dangerous. I mean people could fall, break their neck and kill themselves."

He cited the case of Nelson teenager Finn O'Neill-Stevens, who died after climbing onto a roof and falling through a glass skylight at St Pauls' Cathedral in Wellington last July.

"I just think it's so dangerous just climbing up a tower."

Arrangements were being made for the cone to be removed, Reverend Kirk said.

"It's a bit of a risky job. Anyway, it's got to be done."

Do you know anything about how the cone got there? Contact newstips@stuff.co.nz or phone 03 546 2846