RFS Commissioner Shane Fitzsimmons said the fire in the Royal National Park at Loftus was "clearly suspicious".
The Commissioner said the fire had "multiple ignitions", and crews from the Rural Fire Service, Fire and Rescue NSW and NSW Parks were on the scene fighting the blaze.
"We're working very closely with police and relevant authorities," he said.
In a social media post earlier today Sutherland Shire RFS hinted at the possible dangers of arson attacks in the park.
"The closure of the National Park and heavy presence of NSW Police has been very important," the RFS commented.
"Firefighters from all agencies have done a wonderful job today supported by all Emergency Services and support agencies."
Looking ahead, Sutherland Shire RFS said local conditions would be much milder tomorrow bringing some relief but agencies would remain alert to the dangers.
"The temperature will reach the low 20's with moderate south west winds. The fire danger will be Very High and the Total Fire Ban will remain in force."
The Sutherland RFS posted the following summary of the fire:
"At 6.49pm, a triple zero call was received to a report of a bush fire near Farnell Avenue, in the Royal National Park. Firefighters from FRNSW, NPWS and RFS responded with a significant amount of resources, due to conditions.
"A 'Watch and Act' was issued.
"Crews accessed the fire via fire trails within the National Park.
"Due to the passage of the southerly change, three (3) Strike Teams from FRNSW and RFS responded into the Kirrawee area ready to protect property, this fortunately was not required.
"RFS and NPWS have since contained two fires and are currently blacking out. The fire is now listed as contained and is at the 'advice' level.
"Crews will be working this evening and tomorrow blacking out and patrolling the fire. NSW Police and Rural Fire Investigators will attend the scene tomorrow.
The alert level has been down graded to Advice - there is no immediate danger.
Residents are advised to keep up to date in case the situation changes.
Monitor the situation via the Fires Near Me App or website
Emergency services are responding to reports of a fire in the Royal National Park.
The fire is burning in the area of Farnell Avenue, Loftus, , near Temptation Creek and east of Loftus Oval on the Princes Highway.
The fire is spreading quickly. If you are in the path of the fire, you are at risk.
Fire and Rescue NSW alongside the Rural Fire Service currently have multiple resources on-scene and air assistance is being brought in.
Residents, particularly in Loftus and Kirrawee should monitor the situation as conditions are changing quickly.
The RFS warned the southerly change could push the fire in a northerly direction.
At this stage, the fire is currently under a 'Watch and Act' level but there are reports residents at Grays Point were being evacuated.
Residents need to monitor the situation via the Fires Near Me App or website https://www.rfs.nsw.gov.au/fire-information/fires-near-me
The southerly moved through Sydney, dropping temperatures by 10 degrees in as many minutes and bringing wind gusts of up to 70km/h at Sydney Airport.
Senior Weatherzone meteorologist Brett Dutschke said the "southerly buster" brought "rapid cooling" to coastal suburbs.
Temperatures at Sydney Airport fell from 32.7 degrees at 7pm to 20.5 degrees just 15 minutes later.
"This southerly is really cleansing the air, ridding the dust, ridding the smoke," he said.
Mr Dutschke said the winds would die down overnight, bringing cooler daytime temperatures tomorrow.