An "adventurous" woman likely died from drowning while high on hallucinogenic drugs, a coroner has ruled.

Melissa Deyell Ewings, 31, was last seen on September 20, 2020 near her home at Clarence, 40km north of Kaikoura, where she had been working at a local honey business.

She was acting strangely and her acquaintances believed she was under the influence of drugs.

The next day, she failed to show up for work.

A massive search operation was launched but her body has never been found.

Her death was referred to the coroner by police and this week Coroner Alexandra Cunninghame released her findings.

She found that Ewings died at the Clarence River mouth on September 20, 2020 and the cause of her death was drowning.

"It was an accident, occurring in the context of recent recreational drug use," the coroner concluded.

The coroner heard that after leaving school, Ewings enjoyed travelling and working throughout New Zealand and overseas, with her mother Kylie Ewings describing her as having a "transient lifestyle".

But she kept in touch with her family and was open and honest with her mother about her life.

She knew her daughter had experimented with other drugs including magic mushrooms and synthetic cannabis, and that she had recently taken to making cannabis brownies and other eatables.

In the days leading up to Ewings' death, she was "acting strangely" and friends and neighbours thought she was under the influence of drugs, the report says.

Two days before she disappeared, she texted a picture of black and white spiral-designed squares of paper to a friend, saying she had some acid tabs.

Police told the coroner that at a 2018 event in the Nelson/Marlborough region, a member of the Know Your Stuff drug-testing team had recorded "black on white Aztec spiral pattern" tabs as being "not LSD possibly NBOMe".

The NZ Drug Foundation advises that NBOMe is a "potent group of synthetic hallucinogenic drugs that mimic LSD" and has been linked to many deaths overseas.

It can cause hallucinations, distorted reality, euphoria, strong empathy, panic or nausea, as well as dizziness, hot and/or cold flushes, increased perspiration and numbness in the arms and legs.

On the morning she disappeared, Ewings messaged the same friend, asking, "Do you want to go to the beach today and take acid?"

He said no but she showed up anyway, saying that a famous sportsperson was at the beach.

Looking at her, the friend had a feeling she "must be on acid".

Later that morning, she saw a neighbour outside and asked if he had seen her over in the paddock talking to the cows. He thought she was high on drugs.

She then told another local she had been given a joint which she suspected had been spiked.

Ewings was last spotted on a track at Clarence River mouth at about 7pm.

She was never seen again.

"The narrative of Melissa's last weekend satisfies me that she came into possession of a drug that she believed was LSD, but which may have been NBOMe," Coroner Cunninghame said.

"Over the night of [September] 19 or in the early morning of 20 September she took some or all of this substance... Once at the beach area, she took off her shoes, before entering the water, either the river or the sea, where she drowned."

The coroner added: "It is possible that Melissa would not have gone into the water in the dark if she was not under the influence of drugs. It is also possible that her ability to swim was affected by intoxication... I am not satisfied that the use of drugs was an active cause of her death, although it is certainly very relevant to the circumstances in which she drowned."

In her concluding remarks, Coroner Cunninghame extended her condolences to Ewings' family and many friends, saying she was a fun-loving, adventurous young woman who was friendly and well-liked.

"Sadly, the circumstances of her disappearance have denied her loved ones the closure that comes from being able to farewell her physical self. I hope that these findings can provide some answers."