A pod of 38 pilot whales that washed on a remote New Zealand beach have died after dangerous sea conditions hampered rescue efforts, creating the largest mass stranding in the area in at least 40 years. Conservationists were forced to euthanize 12 surviving whales this Thursday after efforts to re-float them failed. Department of Conservation manager Wayne Costello said the decision was taken in consultation with whale rescue charity Project Jonah and other experts. He said: "Unfortunately the sea on the West Coast is typically rough and the strong tidal currents in the adjoining lagoon made it impossible for rescuers to safely attempt to refloat the whales, allow them to regroup and to try to get them back to the open sea". Costello added in a statement that a rescue "could not be done without considerable risk to both whales and people". Mystery surrounds how the whales became stranded, but experts suggest the currents played a role. There are various factors why this pod came close to shore, we wont know specifically, Project Jonah general manager Daren Grover told Stuff. When they come close to shore they can get caught out by tides, currents and fleeing from predators like orca. More than 150 pilot whales washed up on a Western Australian beach last month, before some were pulled back out to sea using cranes. Experts suggested at the time that a family pod was following a sick member close to shore.