Fire at Kurnell destroys 350 hectares

Fire authorities are investigating whether an arsonist lit a bushfire that has ripped through 350 hectares of land south of Sydney.

Up to 100 rural firefighters worked overnight to bring the bushfire that broke out in the Kamay Botany Bay National Park at Kurnell on Sunday afternoon under control.

The fire has now been contained and downgraded to an 'advice' level, a NSW Rural Fire Service spokesman said on Monday.

The blaze broke out in inaccessible terrain between the water desalinisation plant and the old Carbon Black site near Sir Joseph Banks Drive, according to the RFS.

Fire investigators are on the scene to determine what caused the blaze, which the RFS says was not caused by a hazard reduction burn.

"So it would either be caused accidentally or it was deliberately lit," RFS spokesman James Morris told AAP on Monday.

"It would be very disheartening to see there were people in the community who think they need to put people in the community's lives at risk ... it certainly makes it hard, especially in the conditions we had last night - it could have done some serious damage."

Firefighters responded to the blaze after receiving an emergency triple-zero call on Sunday and battled the blaze overnight in "difficult, windy conditions".

"It was a huge effort from crews on the scene overnight ... now it's just a matter of mopping up," Mr Morris told AAP.