Didrik Hurum, 26, was spearfishing in up to 30 metres of water with four friends when he vanished off North Stradbroke Island just after 10am on Wednesday.

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

Police resumed the search for Mr Hurum on Friday morning after admitting it was unlikely he was still alive.

Eight police divers, two water police boats, two helicopters and a plane are combing the water, while police are conducting beach patrols.

"Police continue to appeal for anyone that may have been in the area and has any information to contact police," officers said.

Brisbane Water Police officer-in-charge Mitch Gray said on Thursday that the 26-year-old may have blacked out underwater.

"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.

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