"The likely cause of death would be the event of the stranding itself and probably suffocation in the sand."
Sperm whales live in the Cook Strait year-round, he said.
Department of Conservation (DoC) staff have been carrying out tests on the whale this morning and have had to cordon it off so people don't touch it.
DoC Wellington Biodiversity ranger Brent Tandy said experts from Wanganui would today attempt to remove the lower jaw bone for local iwi.
"Once that's gone, we'll probably just break it up into bits and bury it. It's too big to move in one piece."
The whale would be cut up with specialist equipment he said. "It will be a bit gruesome."
The tide was coming in, which would bury the whale deeper into the sand, so they would have to wait until it turned this afternoon, Mr Tandy said.
They will need cranes and diggers to move the body.
"Then we'll need to find a section of the beach which isn't an archeological site, because there are archeological sites along the beach among the dunes, so we'll need to find a site that's suitable for burying it.
"And slightly more remote because it will still smell - you can't dig it deep enough to get rid of the smell.''
There were no health issues for swimmers at the beach, but the whale would start to decompose soon, so it was important for the whale to be moved soon, he said.
Local kaumatua Don Te Maipi said it was a very sad day, and it was vital the body was treated with respect.
"It's treated like one of our ancestors."
Iwi had performed a karakia this morning to release its spirit.
This afternoon, before the jawbone is removed, tangata whenua would name the whale.
Local iwi had requested process be done in private.
"It's our taonga," Mr Te Maipi said.
