The Museum Sunfish. The preserved skin of one side of a gigantic Ocean Sunfish (Mola mola) hangs in the stairwell of the Whanganui Regional Museum. Photograph: Whanganui Regional Museum

Some collection items in a museum are like old school chums; you may not catch up with them very often, but it's a comfort to know they're there.

So it is with the Ocean Sunfish that hangs on a wall of the Whanganui Regional Museum.

As visitors go up the stairs they're confronted by this titanic fish.

People can't believe that it's real, or that a fish can look like that.

Many think it must be some kind of giant flounder, but a sunfish swims vertically, its dorsal fin sometimes breaking the water surface like a shark's.

It's the biggest bony fish (that is, non-shark) in the ocean; the largest recorded weighed 2,600 kilograms. It swims by flapping its two enormous fins, and looks like a giant floating head.

The Ocean Sunfish feeds mostly on jellyfish. It gets its name from the habit of sometimes basking at the surface, where birds can peck the parasites off its skin. The Māori word for it is rātāhuihui, and its scientific name,

Mola mola

, is Latin for a millstone.

The museum's sunfish was caught in Napier Harbour in 1895 after it foolishly swam inshore.

The museum's founder and curator, Samuel Drew, was alerted by telegraph from Napier and he offered to buy it. Drew travelled there with fellow scholar Charles Smith. By the time they arrived it had been dead for a week and the locals had cut off many bits as souvenirs. It took five men three days to remove the tough, abrasive skin "that very quickly turned the edges of our sharpest knives". Drew found it a "most unpleasant task to all, and many times I wished we had left it alone."

The pieces of skin were preserved in formaldehyde and reassembled at the Museum in Drews Avenue on a wood and plaster frame. It has been touched up with paint at least twice over the years, necessary for many mounted fish, which discolour and fade with preservation and age. From fin-tip to fin-tip this specimen measures 3.6 metres, almost certainly the largest example of the species in any New Zealand museum.

The sunfish has been on display in the current museum since it opened in 1928 and many people have fond memories of seeing it as children. It will have to be temporarily taken off the wall as part of our earthquake strengthening project Working out how it's attached and bringing it down safely will be quite a mission!

Although earthquake strengthening means the museum's public galleries will be closed from late September to a time in 2018, the Davis Theatre will stay open and available for events. Education and public programmes will continue to operate in a new temporary space in the Post Office building in Ridgway Street which will be fitted out with some exhibitions. The Museum Shop and Gallery will also operate from this site as will some administration services.

When the museum reopens it will have brand new or refurbished exhibitions, environmentally improved collection stores, and of course, in pride of place, will be the Sunfish.

*Dr Mike Dickison is Curator of Natural History at the Whanganui Regional Museum.