The annual event, which has people rushing into the cold ocean, was due to be held on Sunday for its 94th year.
Many swimmers dress up in costume to brave the waves.
St Clair Surf Life Saving Club chairman Cam Burrow said the beach had been "taking a hammering" with the recent weather and a lot of sand had been eroded away.
There was no safe way to access the beach, including through the alternative path by the St Clair Scout Hall and tennis courts.

And the event’s difficulties did not stop there.
With the New Zealand Professional Firefighters Union strike there was also no access to a warm water fire engine, which was an important safety feature for anybody who got too cold.
About 100 people usually turned up and it would be better to postpone the event so it could be run safely.
The event would hopefully be run soon once the access issue was sorted out, he said.