One of the dolphins got stranded and died on Thursday, so conservation staff are monitoring the remaining dolphins to ensure no more become trapped, said Department of Conservation spokeswoman Carolyn Smith.
"We're ... keeping an eye on them," Smith said. "The tide is high, so hopefully they will make their own way back out to sea."
The dolphins swam up the Oruaiti River from Mangonui Harbor on the east coast of North Island on Wednesday, she said.
Killer whales have been spotted recently in Mangonui Harbor and conservation department staff suspect the dolphins headed up river for safety, Smith said. Killer whales, also known as orca, hunt and eat dolphins.
The dolphins were "swimming around and they seem to be OK," she said.
Conservation staff unsuccessfully attempted Thursday to herd the dolphins out of Oruaiti River and into Mangonui Harbor, she said.
A careful count of dorsal fins by conservation staff Friday revealed there were five still surviving in the river.
New Zealand dolphin researcher Kristy Russell has advised conservation staff the dolphins are not in immediate danger of stranding and that a "wait and see" approach is best.
"Too much interference could put them under stress, which could then put them at risk of stranding," Russell said.