A demolition order has been issued for the Timeball Station in Lyttelton.
The stone station was severely damaged by last week's Christchurch earthquake and subsequent aftershocks.
Engineers were called in to assess it on Thursday after it was hit by wind gusts of up to 70km/hr overnight.
New Zealand Historic Places Trust chief executive Bruce Chapman said the decision to dismantle the internationally significant station was made with "enormous regret".
"But public safety is paramount.
"The Timeball Station is too damaged and too dangerous for us to consider anything other than dismantling."
Timeball Station was one of Lyttelton's enduring landmarks, having survived damage sustained in the original 7.1 magnitude earthquake last September.
It was built in 1876 and was one of only five remaining timeball stations in working order in the world.
Flags at the station were used to communicate shipping advice to the town and its ball slowly dropped to signal the time to ships in the harbour.
Mr Chapman said the demolition work on the station would be difficult and dangerous.
Its steep site would present access problems, while its instability could pose a risk to the safety of demolition crews.
"We are not prepared to put anyone's life at risk.
"That said, if we can find a way to dismantle the Timeball Station that allows us to retain as much of the building's materials as possible, we will do so. This site remains significant and we would hope that in future we can do justice to this important building."
He was hopeful Timeball mechanism could be recovered and the tower it rests on rebuilt.
The Timeball Station is of 48 properties nationwide cared for by the historic places trust
It is a Category I historic place - the highest possible grading - because of its maritime history.
Last February, fireman Dave Simpson used the flags to spell out a proposal to his girlfriend Sal.
The couple lived on the hillside beside the station and Mr Simpson said the Timeball was part of their Lyttelton life. His romantic gesture won 'proposal of the year' from a national bridal magazine.
"It was a big part of our view and a part of our Lyttelton. I can't believe it might be gone. We are gutted. It's one of the buildings around here that we identify with and the owners helped me a lot with the proposal," said Mr Simpson.