Bathers hoping to cool off with a coastal swim in recent days might have got more than they'd bargained for, with inshore water temperatures well below average for mid-summer Sydney.

Temperatures at Bondi have been just 16.5-17 degrees for the past few days, said Bruce Hopkins, head lifeguard at the city's most popular beach.

That's well shy of the average for this time of year for Sydney's beaches of about 23 degrees, according to Beachwatch NSW.

Swimmers make a splash off Coogee's Giles Baths on New Year's Day. Credit:Janie Barrett

What's at play is the so-called "Ekman transport" effect - identified in 1902 by Swedish oceanographer Vagn Walfrid Ekman - with wind-driven processes drawing up colder waters along the coast.