'Their objectives are to protect properties, attack and contain the main fire and contain spot fires, ' the ACT Emergency Services Agency said.
The fire is being treated as suspicious with an investigation underway, a NSW Police spokeswoman told AAP.
The park has been closed and people in the park have been advised to leave only if is clear to so or if directed by emergency services.
Given the limited mobile coverage in the Royal National Park, Mr Morris said many people in the bushland were "unlikely" to have received emergency text warnings.
Meanwhile, aircraft water-bombers tried to slow the fire.
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The blaze, which appears to have been deliberately lit on Saturday, had burnt through more than 600 hectares by Sunday morning.
The blaze, which is burning in the Pilliga Forest between Coonabarabran and Narrabri and to the east of Baradine, was downgraded on Sunday evening to advice level.
Firefighters continued to fight the fire overnight, and were successful in extinguishing a smaller fire in the same park just before 9pm.
Adelaide is expecting a top of 31 degrees and Sydney is preparing for the mercury to tip 43 degrees in parts of the western suburbs and 33 degrees in the city, with hot and windy conditions.
Firefighters are keeping a close eye on bushfires across NSW with four regions - the Southern Ranges, Central Ranges, Greater Hunter and Greater Sydney - subject to total fire bans on Monday.