A motorcyclist has been seriously injured in a collision with car in Queenstown's CBD this afternoon.

Police, St John and firefighters were called to the scene, at a roundabout on Man St, at 2.35pm.

A witness said she saw the motorbike "go straight under the car''.

"Everybody was screaming, it was crazy,'' she said.

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"It was really scary.''

She said it appeared the man on the motorbike had tried to turn down a street off the roundabout at the same time as a car was turning.

The motorcyclist could be seen lying face-first on the road following the scene, surrounded by emergency service personnel.

A St John spokesman said the man was taken to Lakes District Hospital in a serious condition.