A Hector's dolphin was found dead on New Brighton beach on Saturday.
DOC aquatic director Elizabeth Heeg said the adult dolphin appeared to have died recently, although it was too early to say what had caused its death.
There were no obvious marks on the dolphin's body. The grisly find was made by a member of the public.
"DOC staff collected the dolphin and have sent it to Massey University. A necropsy will be done to try to find out how it died," Heeg said.
"From necropsies done by the Massey team, we can glean really valuable information about this species and the threats they face."
Heeg said further information received will be uploaded to DOC's Hector's and Māui dolphin incident database and to the DOC website during the next scheduled update in early May.
The database records incidents involving Hector's and Māui dolphins, based on reported events. These include live stranded animals, dolphins found dead on beaches, injured at sea, found floating dead in the ocean or caught as part of recreational or commercial fishing. This data is shared online, along with any necropsy reports, on a quarterly basis.
"Sadly, this is the 12th Hector's dolphin found dead around the South Island since early November 2021," Heeg said.
"At this stage, we don't know why we've had this many Hector's dolphins wash up over the past four months and we will need to await the remaining necropsy results to understand more about possible causes of death."
Eight of these were calves. A very decomposed and scavenged Māui dolphin was also found at Muriwai on Christmas Day but was not collected. Ten of the dolphins have been sent to Massey University. When deceased dolphins are reported and retrieved quickly, they can be chilled and sent to the pathologist.
"They are gradually being examined by the pathologist. We've received results from necropsies on three calves, but a definite diagnosis wasn't possible due to decomposition," she says.
"Maternal separation, potentially during storm conditions, has been cited as the possible cause of death of those three calves."
Results have also been received for an adult female found near Hokitika on November 28, 2021. Age-related disease has been cited as the probable cause of death of this animal. The other dolphins collected since November are still under investigation. Results will be shared on the DOC website when available.
"The information from the necropsies will help us build our understanding of Hector's and Māui dolphins and help with future planning for marine protection and management," Heeg said.
She thanked the member of the public who reported the find: "When people are quick to alert us to discoveries of dead dolphins, it increases the volume and value of the information we can obtain."
DOC urged anyone who finds a dead Hector's or Māui dolphin to report it to DOC via 0800 DOC HOT (0800 362 468). It is also important for the public to report live sightings too via the DOC webpage, or the hotline.