Firebugs burned out almost two hectares of grass and scrub along King Street, Warrawong on Saturday afternoon.
Fire and Rescue NSW inspector John Hawes said the fire was “known to be deliberately lit”.
“As the crews were putting out the fire there were new ones starting up not very far away,” Insp Hawes said.
“It’s guaranteed we’ve got some deliberate firelighters out there.”
Fire crews had to return to the site at least twice on Sunday to deal with reignitions of the fire.