Firefighters needed assistance from the 737 Large Air Tanker Marie Bashir and other water bombing aircraft to combat the blaze on Kangaroo Ridge near Princes Highway, Waterfall.

The highway between Heathcote and Waterfall had to be closed on Saturday afternoon, causing severe traffic disruption, when fire spread from the hazard reduction burn area on the western side to the eastern side. Train services were also suspended.

Sutherland Shire RFS Inspector Scott Deller said hazard reduction burns were risky and were reviewed when they didn't go to plan.

"Introducing fire into the landscape is inherently risky, particularly as we head towards the warmer part of the year," Inspector Deller said in a Facebook post.

"When implementing such activities, multiple controls are applied both in the planning and implementation stage, to reduce and manage the risk," Inspector Deller said.

"One of the main controls utilised is referred to as a 'prescription'. A prescription is a set of conditions or parameters that provide limits or thresholds on whether an activity should proceed or not, based on expected fire behaviour.

"A burn prescription often includes parameters such as fuel dryness, in addition to diurnal weather conditions peaks and troughs, such as temperature, humidity, wind direction and strength.

"Choosing a prescription is a complex mix of the right ingredients - with flexibility applied by balancing and offsetting various conditions, which are highly variable."

Inspector Deller said, regarding "recent events", there would be community scrutiny surrounding the safe implementation of hazard reduction burning, as there should be,

"In any event when things don't go to plan, it will be subject to review by the various agencies who participated. The aim of which is to understand what occurred and to ensure that reoccurrence is avoided in the future."