A rare whale was killed by conservationists last night, bringing a distressing end to a three-week battle to save the mammal.

The 3.7m gray's beaked whale had been a regular sight in the waters off Stanmore Bay on Auckland's North Shore but yesterday beached itself.

Those who had spent three weeks trying to save it were then forced to euthanase it.

Department of Conservation biodiversity programme manager Thelma Wilson said the whale was significantly underweight and getting increasingly emaciated.

It showed no signs of trauma but its behaviour indicated it was seriously ill and its chances of survival were "extremely low".

"There's not much else we could do about it."

Marine biologist Dr Karen Stockin said she had grown close to the whale she had nicknamed Innis after her mother.

Little is known about the breed, which typically grows to around 5.5m long. An autopsy will be held on the whale today.