People launching boats at the boat ramp at Tench Reserve, Penrith. Photo: Wolter Peeters

Sydney faces another scorching day on Tuesday as a big high pressure system steers unusually warm winds over the city.

The mercury climbed to 37 degrees on Monday, the second-hottest maximum on record for this early in the spring and just shy of the 37.4-degree mark set on October 4, 1942, according to the Bureau of Meteorology.

Kids cool off on the rope swing at Tench Reserve, Penrith, as the mercury nears 38 degrees. Photo: Wolter Peeters

The temperature was about 15 degrees above the long-term average for October.

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Sydney can expect another exceptionally warm day on Tuesday, with 33 degrees forecast for the city and 38 degrees in Penrith and Richmond tipped for the west.

"The sea breeze was very, very coastal and we were expecting it to come further inland," Katarina Kovacevic, a bureau forecaster, said of Monday's surprise heat. "There was nothing to stop the temperature rising and rising."

Beach vista at Bronte on Monday morning. Photo: Nick Moir

The city vied the hottest places anywhere in the state, with Penrith's 37.9 degrees and Wilcannia in far-western NSW at 38.1 degrees among the few warmer locations. 

Canberra, meanwhile, posted 31.7 degrees on Monday, the hottest on record this early in the season for the nation's capital. Melbourne, too, posted a new early-season mark, with its 34.4-degree maximum.

The extended heatwave is also likely to set a series of records for consecutive days of warmth this early in the spring.

Hottest October days on record. Photo: Bureau of Meteorology

Ms Kovacevic said the powerful El Nino event developing in the Pacific is one reason for the unusual heat. 

El Nino years, in which typically easterly equatorial trade winds in the Pacific stall or reverse, are often associated with warm, dry conditions across eastern Australia.

Records for warmth are more likely to be broken particularly when combined with the background warming from climate change, scientists say. Along with the warmth, most of Australia has also been abnormally dry recently, with only a few coastal regions recording any rain.

As an example of the dry conditions, Australia average just 0.02 millimetres of rain across the nation in the 24 hours to Sunday morning, a reading perhaps recorded on only a few dozen days, according to Blair Trewin, the bureau's senior climatologist.

A cool change will likely bring some relief from the heat for Sydneysiders on Wednesday morning after another warm night, Ms Kovacevic said.

That change, while lowering maximum temperatures to 23 degrees that day, is unlikely to bring much more than the odd shower over following days.

Almost all of NSW will have a very high fire danger rating on Tuesday ahead of a gusty change, according to the Rural Fire Service.

Temperatures should start to climb again, particularly from Sunday.

The longer term outlook suggests the next bout of hot weather may arrive from the west from about the middle of the month, James Casey, a meteorologist with Fairfax Media-owned Weatherzone, said.

Adelaide can expect temperatures to reach the mid-30s, although Sydney and Melbourne may dodge the worst of that spell, Mr Casey said.

Nine hottest days

The following lists Sydney's nine warmest days on or before October 10, with Monday's top ranking as the second warmest so early in the month in more than 160 years of records, according to the Bureau of Meteorology.

37.4    4 October 1942

37.3    10 October 2013

37.0    5 October 2015

36.7    7 October 1928

35.9    8 October 1936

35.6    10 October 1944

35.2    9 October 1944

35.0    6 October 2000

35.0    10 October 1859