The woman, who was a passenger on the Celebrity Century cruise liner, is believed to have fallen while talking and walking backwards.

The incident happened on Wednesday at Belfast Lough.

An Australian member of the luxury liner's crew dived into the water at Stormont Wharf and rescued the woman before officers from Belfast Harbour Police arrived to administer first aid.

She was transferred by ambulance to hospital where she was said to be in a stable condition.

The drama unfolded just across the River Lagan from where Ian Paisley – on his last full day as Northern Ireland's First Minister – joined Deputy First Minister Martin McGuinness and Scotland's First Minister Alex Salmond to open Belfast's new Stena Line terminal.

A spokesman for Belfast Harbour Commissioners, which employs the Harbour Police, said they were investigating the incident.

And the Northern Ireland Ambulance Service said it had responded to three calls following three separate medical incidents involving the ship's passengers between 9.30am and 1.30pm yesterday.

The 13-year-old, 71,500 tonne, ship – which sailed from Belfast to Dublin last night – can hold more than 1,800.

The liner is currently undertaking a 12-day tour of the British Isles, priced between £2,043 to £3,475 for a couple.

The vast luxury ship – which has nine decks – is one of 36 cruise ships which will visit Belfast this year.

This is an exponential increase since the first cruise ship docked in Belfast in 1997, the year the IRA announced a permanent ceasefire.

Celebrity Century is owned by Celebrity Cruises, a division of Royal Caribbean Cruises, one of the world's largest cruise companies.