The short answer is that these conditions affect much of the NSW coastline,
and are caused by several days in a row of fresh to strong winds.
You need the winds to blow for a few days because it takes a lot to peel off
the top level of warm water and let the deeper cold water rise up. This is
an 'upwelling' and can bring cold water from hundreds of metres below the
surface.
It's called the Ekman Spiral.
Basically, with the winds you've been getting, it means surface water gets transported out to sea. But it doesn't transport the deeper colder water
nearly as much. Result: warm surface water sitting offshore; cold water at
the beach.
See the wikipedia entry for
Ekman Spiral.
We're getting the same old temperatures we always do this time of year up here, no difference.
