The toddler had wandered off from his parents' home on Saturday morning, after going to play outside.

His disappearance sparked a major search of the remote tourist area of Yallingup, before the boy's uncle and grandfather found him, cold and wet in thick scrub.

Mr O'Reilly said there was just "joy and tears" when word came through that James was safe and well.

"I could have ran on water," he told reporters on Sunday.

"It was the highlight of the worst day of my life.

"The fact that he was talking and smiling. He was definitely cold and afraid and hungry.

"It was sad seeing him like that. But he basically walked out of it without a scratch."

James' tearful mother Michelle Buckley said she had feared the worst.

"Once it was getting close to sunset, I was really starting to panic," she said.

"That's what got me upset to think he was out there in just his pyjamas. He would have been freezing."

WA Police said given the time James had spent exposed to the rough terrain and poor weather, arrangements were made for him to have a medical check-up.

The boy had also reported seeing a snake, initially raising concerns of a possible bite.

Inspector Martin Voyez said heavy rain and hail made the search operation very challenging.

Police searched on foot, by air, on horseback and with sniffer dogs all day, assisted by the State Emergency Service, local residents and members of the boy's family.

A drone was also deployed.

WA Premier Mark McGowan said finding James safe and well was "terrific".

"I'm sure all of us who are parents felt for his family in that very traumatic situation," he said.

"Finding little Jimmy was a wonderful thing. A tragedy averted."

Australian Associated Press