This warning follows two incidents at the sand bar in the last two days, the latest of which saw two fishermen being towed back to shore after their engine broke down.
Dunedin marine search and rescue chairman Martin Balch said sand bars were dangerous, unpredictable and required experience to cross.
There have been at least six incidents at the Taieri Mouth sand bar in the past six months. In two cases, boats flipped and people were thrown into the water.
Dunedin police search and rescue co-ordinator Senior Sergeant Brian Benn estimated at least a dozen groups of people had been in life-threatening situations during the summer.
People were being caught out as they did not have time to react when they were thrown into the water, he said.
Long-time Taieri Mouth resident and Otago marine search and rescue adviser Keith Simon said there was a problem with complacency among recreational boaties crossing the Taieri Mouth bar.
There had been few incidents in the two years leading up to this summer and this could have meant people did not prepare correctly, he said.
In the incident yesterday, fishermen Allen Eade (64) and Ted Bailey (84) had set out from Taieri Mouth in a small fibreglass-hulled fishing boat.
They had crossed the bar at the correct time, spent the morning fishing and were coming back to shore about 11am when the main outboard motor cut out.
They managed to get back to the bar using the auxiliary motor, but became stranded on the inside of the bar when the motor was swamped by a wave.
A woman on land saw the men waving for help and rang police, who advised Mr Simon. Mr Simon radioed a nearby fishing boat, Lady Ann, which towed the men back to shore.
"We weren't in any difficulty. But it would have been a different story if we drifted the other way,'' Mr Eade said.
It is understood this is not the first time that Mr Eade has had to be rescued.
Yesterday's rescue follows an incident on Monday when two fishermen were rescued from Taieri Island after their boat flipped on the bar.
Mr Simon said inexperience on the bar could be fatal, especially at present as there was not much of a channel, which meant there were more waves.
