The brother of a Brisbane boatie missing at sea in Moreton Bay for two days says he refuses to give up hope that the young fisherman will be found alive.

Police fear Trent Riley, 26, fell overboard during a solo fishing trip on Wednesday afternoon.

His empty tinnie was found motoring uncontrolled off Mud Island near the Port of Brisbane.

Mr Riley’s friends and family have joined a large-scale air, land and sea search of the surrounding area.

His brother Shannon Riley said the past two days had taken an enormous toll.

“It’s been the worst time of our lives,” Mr Riley said.

“We have massive hope.

“Trent’s a survivor and I don’t want anyone to think [finding him] is not going to happen, because the impossible has happened before.”

Shannon Riley says he believes his brother is on an island or floating at sea. Photo: ABC News/Michael Rennie

‘We’re not giving up’

Missing Brisbane man Trent Riley is believed to have fallen from his boat in Moreton Bay waters.

Police yesterday discovered the missing man’s hat floating off St Helena Island.

“It was a clip-on so he’d actually have to physically take the hat off,” Mr Riley said.

“I believe he was swimming somewhere and the hat was annoying him so he’s just ripped it off and he’s somewhere on an island or the ocean.”

Helicopters, police vessels, the dive squad, Volunteer Marine Rescue and drones have all been involved in the search effort.

Police said yesterday the likelihood of survival at sea decreased significantly after 24 hours.

Mr Riley urged search teams to continue their efforts to find his brother until he was found.

“Anyone who’s got resources, the heart to help, please come here at 7:00am [tomorrow] and we can just do a massive search over the weekend if he’s not found today,” he said.

“People are giving up after two days and saying ‘it’s not possible’ and that’s just not right.

“We’re not giving up — no way.”

-ABC