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The 20-year-old was walking along the breakwater with two others about 5.50pm, when sea conditions were rough.
Sergeant Blair Corlet, of Oamaru, said a heavy wave crashed over the breakwater and swept the man off the concrete structure into the harbour. A member of the public who saw the incident contacted police, but by the time officers arrived the man had hauled himself out of the water with help from the other two men.
He suffered no injuries.
Although instances of people being swept off the breakwater were rare, there had been a handful of cases over the past several years.
Sgt Corlet said people had to take care when they walked on the breakwater in any conditions and advised people to stay off the structure when seas were heavy.
He said the man who was swept off was fortunate to escape injury.
"There have been previous cases with members of the public falling while walking along the breakwater when waves are coming over.
"In this case, obviously, this put resources that could been used elsewhere out of action. But, for us, it is a concern when people are putting themselves at obvious and unnecessary risk."
Signage near the main access points to the breakwater advised people to be wary of rough seas.
In September, the Waitaki District Council voted unanimously to restrict access to the breakwater and lock the gates after dark, following a trial closure.
The closure was in the interest of public safety and to manage crowds that gathered on the breakwater to watch blue penguins come ashore in the area each night.