Two zoo otters who were always together have died of heart attacks on the same night.

Staff at Nelson's Natureland zoo were reeling from the shock of their deaths last week, operations manager Gail Sutton said. However, they were also relieved that they had gone together.

Daz, 19, and Chip, 16, were elderly, as an otter's average lifespan was 13 to 15 years, she said. They had been unwell for two weeks and had spent a week in a quarantine area.

After completing treatment, they were due to return to their enclosure the next day but died that night.

Autopsy results indicated that they suffered heart attacks, she said. Pathology reports were expected to determine what caused the attacks.

"It seems the stress of one having a heart attack was enough to trigger the other heart attack," Ms Sutton said. "They were always so close and never apart. They always cuddled together. Both went together, and that's a blessing for them."

The otters, which were popular with Natureland visitors, lived there for 15 years and helped to raise awareness about the threats facing otters in the wild.

Natureland staff hope to get new otters, and in the meantime will renovate the otter enclosure.