Two men spent almost two hours floating down the Brisbane River from South Bank toward the Story Bridge early Sunday morning clutching a large metal pontoon, police said.

It is unclear if the bizarre cruise was linked to fans of the British and Irish Lions celebrating their 23-21 win over the Wallabies at Suncorp Stadium on Saturday.

However, a source said the men appeared in jovial mood, considering the hour and the predicament, and were speaking with what sounded like British accents. Their flotation device appeared to be a private pontoon broken loose from its mooring, she said.

Police were first called about 1.30am Sunday by people at South Bank who noticed the pontoon floating by.

The entire spectacle was watched by police on the shore, but they were unable to apprehend — or rescue — the men, as the pontoon washed onto rocks around 3.15am near the Story Bridge and the pair disappeared before police arrived.