Deliberately-lit grass fires at Warrawong kept crews of firefighters busy on Monday.
Eight crews from Fire Rescue NSW and the Rural Fire Service battled a number of fires at the Coomaditchie Reserve, which were thought to have been lit by two young people seen in the area at the time.
FRNSW Inspector John Hawes said no property or people were endangered.
"There were several grass fires up there, scattered long distances apart. It was definitely not lightning, there's no doubt these were deliberately lit," he said.
"The area was quite smoky for most of the afternoon. The fire was burning sluggishly, in thick grass and scrub."
Insp Hawes said police were on scene attempting to locate those responsible for the blazes.
The fires burnt out several large pieces of bushland. Most were on the hill's crest, but one burned close to the edge of the Coomaditchie lagoon.
A nearby resident said he had seen four similar fires in the scrub in the past few weeks.
Insp Hawes said the fires were not difficult to extinguish, but said deliberately-lit fires in the area were a constant annoyance.
"It is very labour intensive, and it is remote access to parts of the reserve," he said.
"Anybody who lights a fire runs the risk of being caught in the fire, or having the wind shift and have fire impinge on houses or industry."
The fires were brought under control, and all crews departed the scene by late afternoon.