A police officer was attempting to catch a speeding car when they crashed their motorcycle on an Auckland motorway, seriously injuring themselves.

Waitemata road policing manager Trevor Beggs said the officer was not in a pursuit at the time of the accident about midday on Saturday, but was attempting to catch up to a speeding vehicle.

The crash, on the Southern Motorway, closed one lane and motorists experienced some delays.

A witness earlier told the Herald the officer was lying in the recovery position and was being attended to by passers-by.

The motorcycle was a short distance away with its lights flashing, the witness said.

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Beggs said the officer remains in Auckland Hospital in a stable condition, and is expected to be released in the coming days.

No other vehicles or motorists are believed to have been involved in the crash.

The Serious Crash Unit's investigation is ongoing.