She was shot in the head and Visser said an X-ray, CT scan, and a full necropsy would determine the number of shots and other details.
Seals are protected under the Wildlife Act and Marine Mammals Act, and those harming them face up to two years' imprisonment and fines of up to $250,000.
"No animal deserves to be injured in this way. It would likely have been a very painful death and given it's a protected species, it adds a whole heap of criminality to it," Visser said.
"We ask the community to speak out for the little seal because she can't speak for herself. Someone must have seen something and we urge them to come forward."
She said the matter would be reported to police.
Visser said one clear wound and other wounds were visible, apart from marks that were usually caused by cookie cutter sharks.
Leopard seals were infrequent visitors to Northland shores although they were found around New Zealand all months of the year, she said.
Dr Krista Hupman, also a co-founder of LeopardSeals.com and a marine biologist with NIWA, said it was alarming anybody would hurt an animal that wasn't harming anyone.
She said the incident highlighted the need for more respect for wildlife.
Seals should be given space to rest and people should stay 20m away from them, she said.
Adult leopard seals, which have large heads and jaws and can have a snake-like
appearance, can grow more than three metres long and females weigh up to 500kg.
Information about Friday's shooting or leopard seal sightings can be reported to 0800 LEOPARD, on www.leopardseals.org and on its facebook page.