Firefighters are taking advantage of a rare opportunity to conduct "critical backburning" ahead of dangerous fire conditions predicted for early this week.

Key points: There were over 100 bush and grass fires burning across the state on Saturday night

There were over 100 bush and grass fires burning across the state on Saturday night The Bureau of Meteorology said cool easterly winds would raise humidity on Sunday

The Bureau of Meteorology said cool easterly winds would raise humidity on Sunday Ash was seen washing up on Sydney's eastern beaches as the smog continues to grip the city

The NSW Rural Fire Service (RFS) said it was bringing in extra resources Sunday and Monday to create protective buffers for homes and properties across the state.

A sense of urgency was felt across containment lines last night as fire crews backburned stretches of bushy roads ahead of hot and dry conditions forecast for Tuesday.

With temperatures expected to reach over 40 degrees Celsius later this week, authorities were concerned hot and gusty winds could whip up the "mega fire" burning on Sydney's fringes.

Several blazes combined north of Sydney on Friday to create a 60-kilometre-long fire front stretching all the way to Singleton, on the banks of the Hunter River.

Firefighters work to build containment lines on the Gospers Mountain fire at Newnes Plateau. ( Supplied: Gena Dray )

"A lot of work ahead over the coming days, particularly in anticipation of what is expected to be another heatwave coming into Tuesday," said RFS commissioner Shane Fitzsimmons.

He said the reprieve in the weather would "allow firefighters … to get in and establish containment lines, lots of critical backburning and other techniques".

Last night there were over 100 bush and grass fires burning across the state, with 17 of those at watch and act level.

Black ash and other debris being carried by the blanket of smog blowing in from the spate of bushfires burning across the state made its way to the city's eastern beaches.

Ash and soot were seen washing up on the shores of Coogee on Saturday. ( ABC News: Bindi Bryce )

Sydney woman Kat Schuback said she saw "piles of ash" washing up on the shores of South Maroubra on Saturday afternoon.

"The waves were black as they lapped the shore. We are breathing this in," she said.

Photos taken at Coogee also showed black streaks in the sand by waves as tourists and locals swam nearby in shallow waters.

Bureau of Meteorology (BOM) duty forecaster Rob Taggart said a very high fire danger would remain in place for the Greater Hunter region.

In Greater Sydney and the Illawarra and Shoalhaven regions, the fire danger warning was downgraded to high.

The RFS were backburning along country roads in Mangrove Mountain yesterday. ( ABC News )

Mr Taggart said easterly winds blowing inland from the coasts would give fire crews a window to establish containment lines.

"Generally speaking across most of the state, conditions will be better for suppressing fires than what they [were yesterday]," he said.

"Easterly winds are making it right across the great dividing range and those winds are bringing some humidity to help ease conditions."

RFS spokesman Greg Allan said while the weather had improved from yesterday, residents near those watch and act levels should remain on high alert.

"It's important to realise these are still very dangerous fires," he said.

The RFS have told residents near the Gospers Mountain fire to be ready to evacuate, particularly if they were near Colo Heights, Lower Portland or Upper Macdonald.