It has been removed from the beach by the Department of Conservation for preservation and further study at Auckland War Memorial Museum, Doc said in a statement this week.
Basking sharks are the second-largest fish in the world with estimated reported sizes of more than 12m.
They are filter feeders, with a diet of zooplankton.
The South Westland basking shark measured 3.45m.
The specimen found on Gillespies Beach was a fairly small juvenile.
Records of juveniles and pregnant females are exceptionally rare globally, meaning very little is known of the species’ reproductive biology.
Department of Conservation marine science adviser Dr Karen Middlemiss said it was a valuable opportunity to fill global knowledge gaps on the elusive species.
Auckland War Memorial Museum curator of marine biology Clinton Duffy said until the early 2000s, schools of adult basking sharks, sometimes containing hundreds of adults, were regularly seen around the South Island during spring and summer.
"The abrupt disappearance of these large inshore schools has caused concern for the species’ status in New Zealand and more generally, because New Zealand was considered the hot spot for the species in the southern hemisphere," he said.
"Researchers are hoping the specimen will shed some light on the species’ early life."
The snouts of new-born basking sharks were greatly elongated and had a groove running along the underside to the mouth, Mr Duffy said.
As they grew, the snout became more normal in appearance.
Shark biologists have yet to establish what function, if any, the unusual morphology (shape) of the snout plays in the life and development of the sharks.
"Most juvenile sharks are miniature versions of the adults.
"It’s just one of the many mysteries associated with this species."
Basking sharks are a protected species under the Wildlife Act.
