A woman and her 11-year-old daughter went overboard from a ferry arriving from Liverpool as it moved into dock.
They were rescued from Belfast harbour on Thursday after entering the water from the Stena Horizon vessel, which had departed from Liverpool.
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A rescue boat from another Stena Line ferry that was in the harbour at the time, the Stena Superfast VII, was the first to reach them, and crew members managed to pull them to safety.
They were taken to hospital for treatment.
Stena Line said the incident happened at 6.45pm as the Stena Line was approaching its berth at its Belfast terminal.
A spokesman said: "Stena Line deployed emergency response crews from the Stena Horizon and Stena Superfast VIII, which was in the vicinity at the time.
"Two persons were retrieved from the water and were attended to by local emergency service crews waiting on the dockside in Belfast.
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"Stena Line has informed all relevant authorities of the incident and is currently assisting with the various investigations now under way."
Belfast Harbour Police said they were investigating the incident.
Police said: "Both females were safely recovered from the water and taken to hospital by emergency services."
The Police Service of Northern Ireland said the full circumstances of the incident were still to be established.