Police officers who investigated the death of a scuba diver have concerns that others on the fateful trip have colluded in a cover-up.

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 probe into her death was reopened this week after Coroner Brigitte Windley wanted to further scrutinise witnesses and their differing stories.

Windley was not satisfied with the "reliability of evidence", despite questioning under oath, at an earlier hearing in June.

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

When she resurfaced, she complained that her spare regulator was leaking some air.

The skipper of the boat, an experienced diver, said he managed to fix the regulator before Thongsi descended again.

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, with no air bubbles.

It's not clear who released the air from her buoyancy compensator (BCD).

When it became apparent that Thongsi was in trouble, the skipper took a spare dive tank and plunged down to find her.

He originally told police it was too murky to see anything and shortly resurfaced.

But he later changed his story and said although visibility was up to 3m, he forgot to turn the air tank on and came back up after just 30 seconds. He didn't go back down and after four minutes, he didn't think she would be alive.

It took those on the boat an hour to raise the alarm.

After two more days of evidence in Christchurch – on the back of two days in June - the coroner noted: "There is still so much that is not clear in this case."

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

David Boldt, counsel assisting the coroner, repeatedly pressed witnesses to give details of what happened to Thongsi.

"It's becoming increasingly difficult to get a clear picture of what exactly has occurred here," he told the inquest's final witness today.

Police had "a lot of red flags" over the case, Boldt said, with concerns that the three men on the boat when Thongsi was last seen were "trying to tell a combined story" to account for what had happened.

Today's witness denied lying, but accepted that elements of his story had changed.

He accepted it was "quite possible" that something happened to Thongsi when the skipper and another man were helping her at the side of the boat after she surfaced with a faulty regulator.

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

Police National Dive Squad officers found nothing wrong with Thongsi's equipment, the inquest heard.

When she was found on the seafloor, she had nearly a full tank of air. Her gear functioned perfectly on a reconstruction dive.

A medical expert also found nothing in Thongsi's medical history to explain the sudden death of a seemingly healthy woman.

Windley closed the inquest after the final witness today.

It was likely, she said, that she would make some adverse comment in her findings in relation to those on board the boat.