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Dunedin marine search and rescue chairman Martin Balch said recent floodwaters had completely moved the bar.
A jet-boat which became stranded on the bar about 12.30pm yesterday illustrated the dangers the change presented.
While the boat's occupants managed to get the vessel off the bar without incident, there was potential for much worse to happen and boat-users needed to proceed with great care, Mr Balch said.
The force of the water leaving the Taieri River had wiped out the old entrance across the bar to the north of Taieri Island.
"It is necessary to cross at the south of Taieri Island. But it must be with great care, because it is not surveyed and the water is very muddy, making it difficult to tell where the deep areas are."
The low-tide walking route to the island had also been cut by the shifting sands.
Mr Balch had particular concerns for Queen's Birthday weekend, as the forecast was good and many people were likely to be boating at Taieri Mouth.