Greater Wellington's harbourmaster is warning people to be careful on and in the water after a near drowning at Paremata.

A man is lucky to be alive after he was found floating unconscious in the water under the Paremata bridge.

In mid-afternoon on Wednesday a swimmer found the man floating under the road bridge and brought him to shore where a security guard, Falelua Leitupo, gave one chest compression, checked for breathing and put the man in the recovery position.

The man regained consciousness and Leitupo called for an ambulance.

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Greater Wellington Regional Council contracts the security guard over summer to ensure a safe separation between swimmers and boaties in the busy harbour.

"I think Falelua did a great job managing what was a stressful and potentially tragic situation in a professional and calm manner," said harbourmaster Grant Nalder.

"Yesterday was his first day working at the boatramp – what a way to start. The swimmer has a lot to be thankful for, too.

"We don't have details of why the swimmer lost consciousness, but Paremata is crowded and potentially risky for swimmers in particular. Our advice is - if you are in the water make sure you are visible and when you are on the water always stay vigilant."

The incident follows another earlier that day in Wellington harbour, where a swimmer was hit on the head by a rowing skiff.

That incident happened about 7am off Point Jerningham, when the rowing skiff collided with the distance swimmer, injuring his head.

"Fortunately, it wasn't serious and the swimmer was able to get to shore, but the incident highlights the importance of vigilance on the water.

"All involved were shaken up by the collision and the result could easily have been very different. Both the swimmer and rowers were keeping an eye out but it's really hard to see people in the water."

Nalder urged swimmers to make themselves visible, perhaps by wearing a brightly coloured swim cap or tow bag. Divers should have a large dive flag too, he said.

"Vessels of all types need to keep a good lookout for other water users, big and small, on or in the water.

"As the weather heats up more and more people are swimming, diving and boating on our harbours and off our coasts, creating situations where accidents will become increasingly likely unless everyone takes care."

Nalder said there would be a greater number of swimmers than usual around the harbour training for several upcoming ocean swim events.