A necropsy carried out on the leopard seal found dead on Northland's Ripiro Beach has confirmed it had been shot dead.

Marine mammal expert Dr Ingrid Visser said the team who did the necropsy - a post mortem on an animal - found evidence the seal had suffered after being shot in the head, and that the shot was the cause of death.

The ''sub-adult'', female leopard seal was healthy, in good condition and would have only been resting on the beach, Visser said. The animal had not yet bred but was older than a juvenile.

Read more: Reward offered after leopard seal fatally shot on Northland beach

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The co-founder of LeopardSeals.org, Visser said she could not reveal some information found during the necropsy as it could become part of a police investigation.

The necropsy provided information that would add to scientific knowledge of the species, Visser said.

''Her death will not be in vain as we recovered valuable information about her species, but that does not justify the fact she died in this way,'' she said.

''It was clear from the necropsy that she suffered. We are putting out the call to the community to help the Department of Conservation find whoever did this.''

The senseless killing has outraged wildlife groups and the public, and upset people who came across the carcass near Glinks Gully last Friday.

High-profile marine conservation group Sea Shepherd NZ has offered a $5000 reward for information about the death of the fully protected sea mammal.

''The shooting has been very upsetting for Department of Conservation staff and members of public who saw the dead leopard seal,'' Kauri Coast DOC operations manager Stephen Soole said.

DOC began an immediate investigation because seals are a protected species, and police had concerns about a gun being discharged in a public place, he said. A spokeswoman said police were in the early stages of inquiries.

Before the carcass was removed from the beach, local hapu sang a karakia and named the seal Kohaki, which means ''taken by force''.

Shane Searle found the seal near Glinks Gully, 24km south of Dargaville, on Friday.

He said he had seen plenty of normal seals on the beach but only four or five leopard seals in the last 15 years. Searle, who was enraged at the atrocious act, said someone would eventually spill the beans.

''You don't do that and not brag about it. Five K may loosen a few lips,'' he said.

Information about the shooting or leopard seal sightings can be reported to 0800 LEOPARD, on www.leopardseals.org and on its Facebook page.