As many as 145 pilot whales have died after a mass stranding in Mason Bay on Stewart Island.

A hiker reportedly spotted the whales on Saturday and notified Department of Conservation staff at 10.30pm.

There were two pods stranded at the southern end of Mason Bay, approximately 2km apart.

DoC Rakiura operations manager Ren Leppens said half of the whales had already died by the time they were found and due to the condition of the remaining whales and the remote, difficult to access location, the decision was made to euthanise the remainder.

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"Sadly, the likelihood of being able to successfully refloat the remaining whales was extremely low. The remote location, lack of nearby personnel and the whales' deteriorating condition meant the most humane thing to do was to euthanise."

"However, it's always a heart-breaking decision to make."

Marine mammal strandings are a relatively common occurrence on New Zealand shores, with DoC responding to an average 85 incidents a year – mostly of single animals.

On Sunday 10 pygmy killer whales stranded at Ninety Mile Beach. Two have since died and attempts were being made to refloat the others.

It was the second stranding in the Far North in three days after a 15m sperm whale died on Tokeroa Beach on Saturday, after washing ashore on Friday.

The strandings are unlikely to be related.

Significant whale strandings in NZ since 2010

November 26, 2018

145 pilot whales died after stranding in Mason Bay on Stewart Island.

November 25, 2018
12 whales, understood to be pygmy killer whales, stranded south of Te Paki Stream in Northland. 10 survived.

November 23, 2018
A 15-metre sperm whale died after being stranded at Tokeroa Beach in Doubtful Bay in Northland.

August 5, 2018
2 whales stranded on Baylys Beach near Dargaville. 1 died.

July 9, 2018
2 rare pygmy right whales died at Taupo Bay in Northland.

May 23, 2018
8 sperm whales died after becoming stranded on a South Taranaki beach.

April 5, 2018
32 pilot whales stranded at the mouth of the Okuru River. At least 21 died.

February 10, 2017
416 pilot whales stranded near the base of Farewell Spit, Golden Bay and about 300 died.

November 6, 2015
29 whales died after stranding at Doughboy Bay on Stewart Island.

February 13, 2015
200 pilot whales stranded at Farewell Spit at Golden Bay near Nelson. 140 died.

March 2, 2014
9 orca whales died after stranding at Blue Cliffs Beach near Tuatapere in Southland.

November 5, 2014
60 pilot whales stranded in Ohiwa Harbour in Whakatane. At least 20 died.

January 21, 2014
49 whales stranded off Puponga at Farewell Spit. 3 died.

January 19, 2014
62 pilot whales stranded at Farewell Spit at Golden Bay near Nelson were successfully refloated.

January 15, 2014
15 whales pilot whales died after stranding near the base of Farewell Spit at Golden Bay near Nelson.

January 6, 2014
39 pilot whales died after stranding near the base of Farewell Spit at Golden Bay near Nelson.

November 15, 2012
28 pilot whales died after stranding at the high tide mark on Farewell Spit in Golden Bay.

January 23, 2012
99 pilot whales stranded and more than 80 died at Golden Bay at the top of the South Island.

November 15, 2011
65 pilot whales stranded on Farewell Spit in Golden Bay. 31 died.

February 21, 2011
107 pilot whales died after stranding at a beach near Cavalier Creek on Stewart Island.

September 22, 2010
Seventy-four whales stranded across two kilometres on the remote beach north of Kaitaia. At least 43 died.

August 10, 2010
73 pilot whales stranded at Karikari Beach in Northland and 60 died.

January 25, 2010
50 pilot whales came ashore at Port Levy near Christchurch. 16 died.