A probe into the death of a crayfish scuba diver has been reopened after a coroner raised concerns with differing stories given by witnesses.

Thai national Bua-Ngoen 'Goy' Thongsi, who had been living in Christchurch, failed to surface while crayfish diving off the coast of the South Island in February 2015.

The 37-year-old's body was recovered from Motunau Beach in North Canterbury two days later.

A two-day inquest before Coroner Brigitte Windley was held in Christchurch in June.

But Coroner Windley reopened the inquest after finding she was not satisfied with the "reliability of evidence", despite questioning under oath.

The earlier hearing had failed to provide a plausible and cogent explanation for Thongsi's death, she said today, and therefore she wanted to recall some witnesses who might face "confronting and uncomfortable" questions.

"This is an unusual step and not one I have taken lightly," Coroner Windley said.

An interim non-publication order prevents naming some witnesses and reporting other details of the case.

Two men on the dive trip have given conflicting accounts of what happened during the fateful dive trip.

Thongsi had dived in the water from a boat off the Motunau coast.

She resurfaced after her spare regulator was leaking some air.

The skipper, an experienced diver, pushed the regulator a few times which he says appeared to fix it, the inquest heard.

Another member of the dive party helped Thongsi as the skipper returned to the wheel of the boat.

But as Thongsi went back below the water's surface, witnesses' versions of events differ.

The skipper thinks she was breathing as she went back under, and saw bubbles emerge.

However, another man described her as showing no signs of life, looking through him, and sinking straight down.

Those varying accounts were hard for Coroner Windley to reconcile.

When there were no bubbles seen on the surface, the skipper today said he grabbed a spare oxygen tank and dived to within 3m of the seafloor before realising he hadn't turned the air on.

He resurfaced but did not dive down again and they returned to shore.

David Boldt, counsel assisting the coroner, questioned why it took the skipper nearly an hour before ringing 111.

"There's a lot of emotions going on in my head. I was peed off for a lot of reasons and I'm not going to tell why and I was just trying to think things over," the witness told police.

Asked why he didn't dive again to look for Thongsi, the skipper said that after four minutes without any bubbles, he thought she was dead. The boat would've also drifted away from the spot she was last seen, he said.

"I was just hoping that she would come up," the man said.

"I'm guilty for not going down the second time, if that's what you're saying. I should've gone back down."

On the shore, the skipper allegedly told a volunteer fisheries officer that he'd lost someone from his dive party, and that she had "sunk like a stone".

"I might have over-exaggerated. I can't remember saying it. You can't sink like a stone. You sink, like, naturally," he told the inquest today.

The fisheries officer will also give evidence, Boldt said, that "nothing felt right" about the way the skipper and his mates were reacting about losing a diver.

When asked about that today, the skipper said: "No comment".

Everyone on the boat were "stunned mullets", he said.

"We were devastated."

Asked by Coroner Windley if he'd have done anything different that day, the skipper said he would've phoned emergency services immediately.

Thongsi's body was found just 100m from her last known position at a depth of about 10m.