Didrik Hurum, 25, was spear fishing in up to 30 metres of water when he vanished off North Stradbroke Island just before noon on Wednesday.

His diving companion raised the alarm when they became separated at the popular Flat Rock dive site, off Point Lookout on the island's north eastern tip.

Mitch Gray from the Queensland Water Police has told reporters the young man may have blacked out while under the water.

"I've been very frank with the family with what I believe occurred and it's very unlikely that young Didrik is alive," he told reporters on Thursday.

Police divers and water police are among those still searching for the former member of the Norweigan Armed Forces, who was living in Australia and studying at Griffith University on the Gold Coast.

In March this year, Mr Hurum posted photos of his underwater adventures on Facebook.

"Spearfishing the summer season in Australia," he wrote.

"It's been filled with a lot of good adventures and experiences with awesome people and wildlife encounters. Here is a little summary with Sharks, Marlin, Rays, 9kg Jobfish and so much more!"

Australian Associated Press