A swimmer has been missing at Lake Karapiro since this afternoon.

A 21-year-old man was last seen swimming near Waipuke Reserve, south of Cambridge, when he failed to resurface, police said.

The man was reported missing about 3.30pm and police, the harbourmaster and the Westpac Rescue Helicopter have been searching the area for him.

The man was last seen swimming about 50m from the shoreline at Waipuke Landing, just upriver from the Mighty River Domain.

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A jetski at the scene was initially used to look for the man, but was unsuccessful.

The search for the man has been stood down for the night but will resume in the morning.