Hunt for Sydney bushfire culprit

A blaze that burned through a reserve in Sydney's southwest during a statewide fire ban is believed to have been deliberately lit.

It took firefighters several hours to put out the blaze at The Crest Reserve at Georges Hall on Saturday, with the flames finally doused about 6pm.

Police are calling for anyone in the Rex Road area between 1pm and 2pm who saw suspicious activity to come forward.

A total fire ban is in place in NSW during a week-long state of emergency as firefighters respond to the ongoing bushfire crisis.

Meanwhile, police on the NSW South Coast are reminding residents they will be investigating fire-damaged sites on behalf of the coroner to find out the cause and impact of devastating bushfires in the area.

Detective Inspector Scott Nelson said the process may take up to three weeks or longer depending on the weather.

"As part of our inquiries, we'd also be keen to review footage or images anyone has on their phones or other devices, that show any of the fires in their infancy, even if from a distance," he said in a statement.