Sydneysiders woke up to their coldest July morning in 21 years today, when the thermometer dipped to 3.7 degrees.

The minimum temperature was reached at 6.54am today and beat by one degree a July record set just yesterday.

"We had high pressure sitting over the state so, with a clear sky and very little wind, here it comes, the lowest temperature," senior meteorologist at the Bureau of Meteorology, Peter Zmijewski, said.

The temperature is the lowest recorded at Sydney's Observatory Hill since July 27, 1986, when the mercury plunged to 3.1 degrees.

There is no chance of snow falling in the city but the Blue Mountains may soon be dusted in white.

"It is possible we will get snow in the Blue Mountains above an elevation of 600 metres tomorrow," Mr Zmijewski said.

Early risers wrote into smh.com.au about frost-covered gardens in Hornsby Heights, rowing in Balmain on water like a millpond and running on ovals of crunchy grass.

One reader, Tania Liebenow, knew the morning would be cruel when her cattle dog wanted to stay in.

"You know it's cold when your Australian cattle dog is whining for you to dress her in her hooded fleece doggie jumper and refuses to go outside in the morning," she said.

The cold snap came as about 3000 homes lost power in North Sydney, Cammeray, Eastwood, Epping and Hornsby after an underground cable fault caused a blackout.

Some homes went without heat and light for more than six hours.

The RTA's Traffic Management Centre said traffic lights in Cammeray, Crows Nest, Naremburn and Eastwood were out between 7pm and 8.30pm.

AAP reports: West of the city, the mercury was much lower, at minus 3.7 degrees in Richmond, but did not come close to breaking any records.

Despite the icy low this morning, forecasts suggest a pleasant day with tops of 16 degrees in the city.

As winds pick up, tonight is expected to be milder than last night, with a warmer morning tomorrow but a colder day later.

The bureau has forecast snow and sleet today for the Southern and Central Tablelands (including the Blue Mountains) above 500 metres.

Blizzards are also expected over much of the NSW snowfields this afternoon.