Department of Conservation says the stranded humpback whale near Dargaville will be euthanised after rescue attempts have failed.
The female whale, which was found with a now dead calf, has been stuck on Ripiro Beach since early Sunday.
More than 100 rescuers tried to refloat the whales during the past two days to no avail.
DOC said it made the decision with local iwi and Project Jonah because her condition was deteriorating.
Whale specialist Ingrid Visser, who is at the beach, said the decision to euthanise was never made lightly.
"During the night, the whale was lively but was being buffeted around by the waves," she said.
"This morning she has definitely dropped in her ability to be able to extricate herself from the situation that she's in."
DOC said the beach will be closed to the public when the whale is euthanised, and formal processes, including blessings and a karakia, will be followed.
'Grim' prospects for stranded whale at Ripiro Beach - mayor
Earlier today, Kaipara Mayor Jason Smith told Morning Report the prospects of refloating the whale were not looking good.
"There's a bit of sea fog and the sun hasn't yet risen but things are looking grim actually. The whale has been on the beach for more than 48 hours and so the prospects are not good," Mr Smith said.
There had been a lot of community effort to help, with a project team led by Dr Visser, Project Jonah, DOC and whale rescuers all on board yesterday, but the time for that was over, he said.
"I think right now the time for the community volunteers who have really pitched in and tried, with diggers and on ropes... the time for those volunteers is now over."