Mystery surrounds the death of two Hector's dolphins found washed up on a beach near the Waimakariri River mouth in north Canterbury this week.

The Ministry of Agriculture and Forestry is investigating after a member of the public found the pair entangled in a fishing net, and reported the find to the Department of Conservation.

The use of set nets is banned under the Fisheries Act for four nautical miles along the entire east coast of the South Island to protect the endangered species, suggesting the deaths are the result of illegal fishing.

Ministry spokesman Scott Gallacher said the fishing net found around the dolphins was thought be a recreational one.