The retired Perth vet who helped rescue 12 boys and their coach from a flooded Thai cave was pessimistic the massive effort would be successful, his partner says.

Craig Challen was packing to go on a holiday to the Nullarbor with his diving buddy Adelaide anaesthetist “Harry” Harris on Thursday last week when his friend called and changed their plans.

The pair were on their way to Thailand within an hour, with Mr Challen’s partner saying there was no way he was not going to help.

“I did express some concerns, obviously, but he was pretty adamant,” Heather Endall told ABC radio. “There was no talking him out of it.”

Richard Harris (centre, back) with dive partner Craig Challen (left of him) with medical staff caring for the rescued boys in Chang Rai Hospital. Photo: SAAS

She said he was delighted with the outcome but initially doubted the huge undertaking would end well.

“He did hold concerns for the whole situation,” Ms Endall said.

“I think he went over there with a bit of a pessimistic view – he thought it was going to be a real challenge to get the boys out alive.

Just the predicament in which they were in, and how long they’d been there, and the cave itself, I think he felt it was going to be a tough call.

“He didn’t actually display any regard for his own safety.”

Mr Challen and Dr Harris are key members of an elite Australian diving group called the Wet Mules – a name inspired by the expression “enough money to burn a wet mule”.

Footage has emerged of the men emerging from the cave with Thai rescuers, highlighting the bond cemented by the experience of the past three weeks.

“As a large part of our chosen pursuit of cave diving seems to revolve around ferrying heavy objects in and out of caves, submersing ourselves in frigid waters for many hours and generally abusing our bodies in a multitude of ways, we were beginning to take on the persona of the wet mule itself,” the Wet Mules website says.

“Stubborn, strong of back and oblivious to pain: these are the qualities of the exploration cave diver.”

New South Wales diving consultant David Strike told the ABC he was not surprised Dr Harris had asked Dr Challen to help him on the Thai mission because close personal relationships were vital in difficult dives.

“Like most divers, you tend to get used to a particular diving partner,” Dr Strike said.

The pumping system used to drain water from the cave system. Photo: Royal Thai Navy

“Telepathy almost comes into play. You become very comfortable with a particular dive buddy, you know how they’re going to react in a certain situation.

“Just the predicament in which they were in, and how long they’d been there, and the cave itself, I think he felt it was going to be a tough call.”

Ms Endall told the ABC her partner would not have seen the dive as personally dangerous, despite the earlier death of a Navy SEAL underground.

“They’re all very humble … they don’t like to promote how good they are but they’re all exceptional guys,” Ms Endall said.

She described her partner as quiet and serious, and a high-achiever.

“He flies, he dives, he mountaineers, [he’s] exceptionally intelligent,” she said. “He’s very kind hearted, he’s very thoughtful, he’s very much up to date with world events.”

But she suspected Dr Challen would shy away from the spotlight his involvement in the rescue would bring.

They might just nick off to the Nullarbor to hide down a cave from the media attention … I suspect he would like to.”

The two men have gained many years of experience together, pushing the limits of some of the most difficult caves, such as Cocklebiddy Cave in Western Australia and the Pearse River on New Zealand’s South Island.

In a book entitled Dining with Divers recently published by Mr Strike and Simon Pridmore, Dr Harris wrote of the “terrible fright” he had when Dr Challen took longer than expected to return from the far reaches of Cocklebiddy Cave, considered one of the most beautiful in the world, in 2008.

“I’ll give him five more minutes, I thought, and then I’ll have to go,” he wrote.

“Suddenly I saw a glimmer of light and there he was in front of me, empty reel in hand.”

“We were high-fiving and whooping and hollering underwater.”

Dr Challen told National Geographic magazine about a scary moment in their record-breaking attempt to reach the source of the Pearse River, thought to be the deepest cold-water cave, in 2011.

“I noticed going down, and I just started breathing really heavily,” he said.

“It’s really like a self-perpetuating thing, you know. You try and relax and chill, and you just can’t.”

Mr Challen co-founded the Perth veterinary franchise Vetwest but retired from the profession last year and now describes himself online as a dilettante.

Prime Minister Malcolm Turnbull told the ABC that Dr Harris had spoken of the extraordinary heroism of the boys and divers, which would stay with him.

“I am inspired by the remarkable international cooperation,” Mr Turnbull said.

“You know, you look around the world there’s a lot of tension. What an extraordinary thing.”

-with ABC, AAP