Beachgoers flocked to the sand and surf to make the most of sunshine and sweltering temperatures that were nearly 10 degrees higher than average for March by about 1.30pm.

The mercury dipped to 20 degrees in the city and 15 degrees in the western suburbs overnight before temperatures began to rise steadily on Sunday morning.

The weather warmed up to 40.1 degrees in Bankstown by 3pm, while the CBD hit a high of 38.2 degrees after 5pm.

In the western suburbs, it was 38.8 degrees in Penrith, while Badgerys Creek notched 40.0 degrees and Camden reached a stifling 39.7 degrees.

Weatherzone meteorologist Kim Westcott said the mercury was expected to peak late in the afternoon.

"We're looking at a pretty even spread of temperatures across the city, about 40 to 41 degrees basin-wide. So there's no relief at the beach.

"It will still remain quite hot until about 7pm or 8pm and will probably be quite a hot evening."

Ms Westcott said Sunday's heat brought temperatures well above the long-term average for March of 25 degrees.

The Sydney CBD was also edging towards its hottest March day - which was recorded at 39.8 degrees on March 9 in 1983 at Observatory Hill - in about 150 years of records.

"Maybe we will see a record today," Ms Westcott said.

A record-breaker was slightly less likely in the western suburbs, where the hottest March day in the past 20 years of records was 40.6 degrees at Penrith in 1998.

Meanwhile, the NRL game between the Parramatta Eels and the Manly Sea Eagles has kicked off at Brookvale Oval on Saturday afternoon in temperatures above 38 degrees.

The burst of heat had been caused by a large volume of hot air in the country's interior that was sweeping across NSW, according to Ms Westcott.

While Monday was expected to be warm, temperatures were expected to drop to about 23 degress for the second half of the week, bringing cloudy skies and showers.

"If people want to make the most of the sunshine, now's probably the time," Ms Westcott said.

The NSW Rural Fire service issued a total fire ban covering more than half of the state, including the Greater Sydney Region, Greater Hunter, Illawarra/Shoalhaven, Far South Coast, Monaro Alpine, Southern Ranges, Central Ranges, North Western, Lower Central West Plains, Southern Slopes and Eastern Riverina regions.

At 2.30pm, there were 27 bush and grass fires burning across the state, with 17 uncontained.

The NSW Rural Fire issued an "emergency" warning for a blaze burning in bush close to Bega shortly before 4pm. The fire is burning under "strong and gusty north-westerly winds", and is threatening properties in Tathra.

Residents in the Thompson Drive area have been told it is "too late to leave".

A "watch and act" warning was issued on Saturday in relation to another fire burning in Bega, close to isolated properties, but firefighters have slowed the spread of the fire.

Meanwhile firefighters are "gaining the upper hand" on a bush fire burning close to a number of isolated properties at Greenwich Park, near Goulburn. A "watch and act" warning remains in place at 4.30pm.

Strong winds sweeping through the ACT amid recent dry weather conditions dragged a dusty haze across Canberra on Sunday afternoon.

In Victoria, homes were destroyed as firefighters battling fierce winds worked to contain a cluster of fires burning out of control in the state's south-west.

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