North Island

The sea in North Island peaks in January, when the water averages around 20°C.

The main swimming season runs from January – February, when surface temperatures stay comfortable for extended sessions.

On average, 2 months a year sit above 20°C — the threshold most swimmers describe as pleasant without a wetsuit.

Shoulder months like December and March are noticeably cooler but often quieter, with fewer crowds and steadier weather.

Outside peak season the water drops toward 13°C, so a 3/2mm shorty or full wetsuit makes long swims much more comfortable.

The coldest month is typically August at around 13°C, when cold-water swimmers dominate the coast.

North Island sits in the Southern Hemisphere, so the warmest sea coincides with the December–February summer and the coolest with July–August.

Long-term climatology for North Island shows an annual mean sea temperature of 17°C, ranging from 13°C in winter to 20°C at the summer peak.

The seasonal swing is roughly 7°C between the coldest and warmest month — a useful yardstick when comparing years or checking whether the current sea is running above or below average.

Recent decades have trended slightly warmer at North Island, in line with regional sea-surface temperature records for the wider basin.

Monthly averages below are drawn from multi-decadal climatology and are updated as new observations arrive.