Hope is fading of finding alive a Chinese PhD student believed to have been swept out to sea off rocks at a popular marine reserve north of Auckland on Sunday.

Yue Gui, 30, known as "Alice", was last seen at 11pm on Saturday at the Leigh Marine Reserve Faculty, operated by Auckland University, about 80km north of Auckland.

Sergeant Bede Haughey of Warkworth police said it was feared she fell into the water off rocks while doing a routine ocean temperature check.

"We had a four- to five-metre tide through there on Sunday morning so it is consistent with her being swept away from the location," Mr Haughey said.

It was not until that afternoon that it was noticed she was missing, along with the water temperature-measuring apparatus, while all her belongings were still in her room.

Mr Haughey had spoken to her next-of-kin through an interpreter and they were coming straight from China.

"They were quite traumatised but they're hopeful that there will be a positive outcome to the search, but they're aware that as time progresses the chances of Alice being found alive diminish."

About 10 police dive staff and a police helicopter are involved, along with civilians combing the shore.

Mr Haughey said the search would continue today and into tomorrow, if need be.

Ms Gui was about 1.55m tall, of slim build and with dark hair.