A police motorcyclist is seriously injured after coming off his bike on an Auckland motorway.

Police said the officer came off his bike on the southern motorway just after midday.

One lane was closed off at the port ramp. NZTA has warned motorists to expect delays as lanes were also blocked.

A witness said the officer was lying in the recovery position and was being attended to by passers-by - while the motorcycle was lying a short distance away with its lights flashing.

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No one else is believed to have been involved in the accident and serious crash unit is investigating.

The officer remains in hospital in a serious condition.