South Island

The sea in South Island peaks in January, when the water averages around 15°C.

On average, 0 months a year sit above 20°C — the threshold most swimmers describe as pleasant without a wetsuit.

Outside peak season the water drops toward 9°C, so a 3/2mm shorty or full wetsuit makes long swims much more comfortable.

The coldest month is typically July at around 9°C, when cold-water swimmers dominate the coast.

South 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 South Island shows an annual mean sea temperature of 12°C, ranging from 9°C in winter to 15°C at the summer peak.

The seasonal swing is roughly 6°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 South 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.