Police have scaled back their search for a missing UK tourist who jumped off an inner-city bridge into the Brisbane River, in what they have described as a "drunken stunt".

The 30-year-old man had been with a group of friends when he leapt off the William Jolly Bridge about 10:30pm on Sunday.

Police Inspector Sean Cryer said four men who had been drinking together at a local backpackers hostel agreed to jump off the bridge for "a bit of fun".

"The young man in question ... jumped off the bridge first," he said.

"The other three fellows said he took a running jump from three or four metres away — jumped over the rail.

"They observed him go into the water, he came back up again a few seconds later, rolled on his side, went back under the water and he hasn't been seen since."

A major overnight search failed to find any trace of the missing man.

Police also conducted air and land searches all day on Monday, but by nightfall, had still not found the man.

The search will resume on Tuesday.

"We'll continue our search and rescue search until we find him," Inspector Cryer said on Monday morning.

"Obviously jumping off a bridge into a river is a silly thing to do, and we'd warn against it."

The man had been working in Australia.

Police have contacted the man's parents in the UK but were still trying to notify relatives in Brisbane.