Total fire bans are in place for Monday in certain areas, with much of NSW experiencing hot and dry conditions.

The Wollongong, Kiama, Shellharbour, Wingecarribee and Shoalhaven council areas have a very high fire danger rating.

The NSW Rural Fire Service said on Sunday afternoon that conditions were worsening as 700 firefighters battled 33 bush and grass fires across the state.

The Royal National Park will remain closed on Monday with access only for residents.

Two fires have burnt through more than 1800 hectares of the park. Firefighters worked through the night to extinguish one of those blazes and were attempting to bring the second fire under control before Monday's worsening conditions.

Read more: Bushfires turn Wollongong skies orange and black

Hundreds of visitors to the park were rescued by boat from beaches after becoming stranded on Saturday. 

"For those that left vehicles at Bundeena yesterday. You can return using the Cronulla Ferry service and drive out of the park," the RFS tweeted on Sunday.

The blaze is being treated as suspicious with specialist investigators heading into the park on Sunday, a NSW Police spokeswoman told AAP.

RFS Deputy Commissioner Rob Rogers said it was lucky nobody was killed in the blaze and it was infuriating that someone could think of starting a fire on purpose.

"It is absolutely frustrating as I think the average person in the community finds it's just unbelievable that people would do that.

"Given there was more than one fire obviously we are concerned about how the fires started," he told the Seven Network. 

Mr Rogers said it was lucky no one killed, but it could easily have been a very different story if it wasn't for all the agencies working together. 

"And look at the cost - this firefighting effort will cost millions of dollars. There is a cost of these things, let alone that trauma to the individuals," he said.

NSW has endured hot, dry temperatures for several days, with Sunday marking the fourth consecutive day above 40 degrees for parts of the state.

While the CBD kept to a top of 27 degrees thanks to a gentle sea breeze, in Sydney's west the mercury notched 40.3 degrees shortly before 4pm - marking the area's fourth consecutive day above 40 degrees.

Across the state, five bush or grass fires were still being brought under control on Sunday evening and three fires were burning out of control, although they were at the lowest alert level.

Firefighters are fighting an out-of-control bushfire near Bannaby under "variable winds" on Sunday afternoon, which is threatening isolated properties.

Fire crews are working with residents to prepare properties and establish containment lines for the blaze, which is over 1600 hectares in size.

Further north, a bushfire in the Pilliga Forest near Narrabri has burnt through more than 51,000 hectares with aircraft being brought in to help control the blaze.

Earlier, the RFS has issued a "watch and act" warning for residents as the fire burns in a south-westerly direction towards Dandry, with the Newell Highway closed between Narrabri and Coonabarabran due to the fires. On Sunday night it was at the lowest level of "advice".

A fire in Bundarra, north of Tamworth, was out of control, having already burnt through more than 10,000 hectares and threatening isolated properties.

A truck crash just south of Cowra sparked a grass fire that was threatening some nearby rural properties just after 1pm.

Aircraft were called in to help ground crews but locals and firefighters were able to bring the fire under control just after 1.30pm.

With no chance of rain until late in the week, firefighters can expect another tough week trying to contain bushfires across the state.

The RFS has issued a total fire ban for the greater Sydney region on Monday, as well as the Central Ranges, Southern Ranges and Greater Hunter regions.

Little relief from the warm weather is in sight for Sydneysiders, with Monday heading for 33 degrees in the CBD and temperatures expected to remain around the 30-degree mark all week.

In Sydney's west, the mercury is expected to hit 43 degrees on Monday, potentially marking a fifth day above the 40 degree mark.

Temperatures out west are expected to remain in the high thirties with "clear skies" for the rest of the week, according to Drew Casper-Richardson, meteorologist at Weatherzone.

"We've got a stable weather pattern stopping any weather changes coming through - a low pressure trough is dragging that hot weather in from the centre of Australia," said Casper-Richardson.

"So any changes coming through will be fairly weak, at this stage. 

"There's a chance of a storm and some rain from about Friday - that might bring a bit of rain, but it will make it more humid. So there's a bit of a catch-22 there."

with AAP

Weatherzone is owned by Fairfax Media, publisher of this website.