People are believed to have jumped into the water for safety after a boat caught fire in Lyttelton Harbour.
The blaze broke out around 12.45pm on Saturday, close to Quail Island, and black smoke could be seen from across the harbour.
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Seven people were on the boat at the time, but no-one is believed to have been injured.
Fire crews and Royal New Zealand Coastguard used a barge to reach the boat and extinguished the flames using foam and a portable pump.
The vessel sank and will be retrieved next week.
A mayday distress call was made but the boat’s emergency beacon was not activated.
The events were witnessed by Church Bay resident Graeme McColl who said the boat appeared to be “quite a large runabout, with a cabin”.
“We looked out and saw this plume of smoke and immediately got the binoculars,” he said.
“It was smoking and in flames. There was black smoke for ages, so there must have been a lot of fuel on board.
“Quite a few jet skis and little boats were circling to keep an eye on it, but not too close because it was pretty dangerous at that stage.”