A fierce blast of winter has prompted a severe weather warning for most of southern NSW, including Sydney, that will remain in force until Wednesday.

The Bureau of Meteorology is forecasting damaging winds for an arc of the state stretching from the border with South Australia and Victoria all the way to Newcastle. (See map below.)

Gusts on the peaks of the southern ranges above 1800 metres may reach 120 km/h on Tuesday. By Wednesday, winds averaging 50-70 km/h and exceeding 90 km/h at times are likely for the southern and central ranges and nearby coasts, the bureau said.

A bg cold front has southern Australia in its grip. Photo: Paul Jeffers

For Sydney, the maximum temperature will plummet from a mild 20 degrees on Tuesday to 13 degrees by Wednesday as the powerful cold front sweeps across NSW.

"With the progress of this front, we'll have a pretty decent wind chill," David Grant, a duty forecaster with the bureau, said. "It will feel like it's single digits."

Wind gusts should ease by Wednesday afternoon but will still be blowing, with sustained speeds of 20-30 km/h by the evening when fans flock to Homebush for the State of Origin league match between NSW and Queensland.

"Even though it's a westerly wind, [the chill] can catch people by surprise," Jacob Cronje, a senior meteorologist with Weatherzone, said. 'It's going to be cold all day."

The chill won't loosen its grip on Sydney - and most of the rest of the state - until the weekend.

Behind the front, a big high-pressure system will settle in, leading to clearer skies but frosty nights.

Sydney had an overnight minimum of 14.8 degrees on Tuesday, nearly 7 degrees above average.

Mild overnight conditions will bring lows of 9 degrees on Wednesday but a sharply colder 6 degrees on both Thursday and Friday morning, the bureau said.

For inland regions, such as Richmond, the mercury will dive to 1 degree on Thursday and to zero on Friday.

Snow is expected to fall to 300 metres on the southern ranges on Tuesday and to 600 metres on the central ranges on Wednesday, Mr Grant said. West-facing slopes will get the deepest falls.

Orange, in the central west, has a chance of getting some snow on Wednesday, Mr Cronje said.

One feature of the cold front is not just its strength but the fact "it's penetrating quite far north", Mr Cronje said.

The clouds over Australia are also attracting interest among meteorologists for their reach - stretching from Sri Lanka to across the Tasman.

Gale warnings are also in force on Tuesday for coastal waters from Eden to the Hunter, with gusts reaching 95 km/h on Montague Island on Tuesday.

Similar conditions are expected on Wednesday, the bureau said.

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