UPDATED 4.56pm: The Kestrel Preservation Society is trying to get to the bottom of how the historic ferry sank at a wharf in Auckland.

It went down at Wynyard Quarter some time overnight.

The Kestrel is more than 100 years old and is considered by many to be the city's most significant maritime icon. It was built at the site of the Tepid Baths in 1905.

It was the last working double-ended steam ferry in the world.

Spokesman Hugh Gladwell said the boat had been hauled and surveyed about four years ago and was deemed to be in good condition for five years.

He said it had just got an electrical warrant of fitness, had a good bilge-pumping system and was inspected regularly.

"So it's a complete mystery to us. It was floating well at nightfall last night, and this morning it was gone."

Hugh Gladwell says divers have been called in to inspect the boat, but said it's unlikely the historic ferry can be put back together again.

He said it's a very sad loss of an important part of Auckland's maritime heritage.

A trust had been raising money to restore the Kestrel but it's not clear what the condition of the vessel will be now.

Sarah Bloomfield, who set up the trust to save it, she said as much as they tried, the group couldn't grab the public's attention.

"I think she's more worthy than elephants for zoos and artworks on the end of Queen's Wharf.

"So many stories of the romances that were started there, the person who fell. She went out to ferry ashore the original All Blacks when they came back from France".

Auckland Councillor Chris Darby hopes the city icon can be recovered.

"These are pretty hardy vessels. I'm sure all the electrics and electronics will be severely damaged but would imagine that whole of this vessel that's been ploughing the Waitemata for a long, long time will be pretty robust".