A train fault at Britomart station is causing major disruptions to Auckland's rail service this morning.

The rush hour commute on the busy rail network has been thrown into chaos with passengers being told by Auckland Transport to expect delays and cancellations across all lines.

All trains on the Onehunga line have been suspended.

Auckland Transport said a rail shuttle service was now ferrying passengers on buses between Penrose and Onehunga.

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An Auckland Transport spokesman said a train had broken down at Britomart station and there were difficulties moving it from the station at rush hour.

Trains heading into the city station were experiencing trouble trying to get into the underground station which was adding a further delay to today's journey.

Timetables across the network had been slashed with trains expected to arrive every 20 minutes on the western, southern and eastern lines.