Water police are searching for a man in the Brisbane River, southwest of the city, after reports he failed to resurface while swimming.

Water police are searching the Brisbane River after reports of a possible drowning.

Police were called to Kookaburra Park in Karana Downs, southwest of Brisbane, about 2.15pm (AEST) on Sunday.

A man is believed to have been in a group near a boat ramp when someone saw him go under the water and fail to resurface.

Swiftwater rescue technicians in inflatable rafts were also assisting with the search, a Queensland Fire and Emergency Services spokesman said.