NSW boils in 40degC heatwave

New South Wales is sweltering in a heatwave that's seen temperatures soar into the 40s.

It's 38degC in downtown Sydney and even hotter in the city's western suburbs.

At Taronga Zoo, the animals have been beating the heat with an icy breakfast, with fish- and blood-cicles for the otters and meerkats, and grape-cicles for the red pandas.

There's a simple answer to surviving the heat for those lucky enough to still be on school holidays - swimming and eating ice blocks.

But there's also a warning.

The beach is the place to beat the stifling heat, but health authorities say ozone pollution levels will reach their peak around 7pm on Wednesday (local time), and asthma sufferers should stay inside.

It's so hot even Australian Prime Minister Malcolm Turnbull weighed in, tweeting: "Please stay cool, keep an eye on elderly neighbours and children".

The heat's also an issue at the international tennis at Olympic Park, where play can be suspended under extreme heat rules.

But most working in the city have to soldier on.

The hot conditions are expected to ease on Thursday, but Friday will be just as hot.

Newshub.