The New South Wales Rural Fire Service (RFS) says koala protection has been at the "forefront" of planning for a major hazard reduction burn in Sydney's south west over the weekend, but environmental groups remain concerned.

The burn was supposed to start today, but the RFS has announced on social media the burn has been postponed due to rain and is scheduled to start tomorrow.

The area is also known habitat for a growing koala population believed to include at least 180 of the endangered animals.

Illawarra/Sutherland RFS acting manager David Barnett said a lot of planning had gone into preparing for the burn and it would be conducted along guidelines developed for threatened species.

"So that means having a fire [with] a half to one-metre flame height and trying to keep … the upper canopy out of the fire," Mr Barnett said.

The burn will be conducted from the northern edge of the identified area, southwards down each side.

"What that does is it allows the koalas and other wildlife … to move out of the path of the fire — hopefully without getting burnt or anything — giving them the opportunity to get into refuge areas as well," he said.

The RFS will use an aircraft with an infra-red camera to conduct a thermal heat survey.

Firefighters on the ground will also use the cameras to conduct visual inspections.

"So if they see any koalas, their objective is to hold the burn until the koalas are able to move on – we won't burn them out – we have got them front of mind."

RFS preparation for the burn has been limited, as an area of the barracks contains unexploded ordinances where prior inspection has not been possible.

Defence has also prevented the RFS from using drones to survey for the animals.

A National Parks and Wildlife team and trained RFS members will provide any injured wildlife they come across with assistance.

Mr Bartlett said the burn was an important part of preparing for the bushfire season.

"It's going to provide a significant buffer, if we get this in, to the communities of Engadine and Heathcote," he said.

Koala numbers in barracks unknown

Sydney Basin Koala Network manager Stephanie Carrick said she was very concerned by the prospect of the burn.

"The area has not been surveyed for koalas, we don't know how many are in [the barracks] and if they have been surveyed, they have not made that public data," she said.

"So it is really difficult to know how the koalas are connected to the animals on the Campbelltown side and Sutherland Shire side."

She urged the RFS to take great caution with the burn.

"[The koalas] do still have to come down the tree to move around, they don't move from tree to tree, so I hope they are identifying the koalas before they start any burns," Ms Carrick said.

"Of course we understand that these mitigations are needed to protect people, but we do want it to be done in the most sensitive way for this critically important koala population.

"This population could be key to the recovery of the species across the state."

Tense times on Heathcote Road

Ms Carrick said the size of the burn would most likely cause the animals to flee onto the busy nearby Heathcote Road.

But on Thursday, Heathcote MP Maryanne Stuart announced new koala signs would be installed in the area.

Transport for NSW is expected to install six permanent signs and two temporary koala signs between the Princes Highway and Deadmans Creek.

Catherine Reynolds from the Sutherland Shire Environment Centre said there had been ten known koala deaths on Heathcote Road since October 2022.

"And one this week on Tuesday morning," she said.

"We welcome the new signs being installed and have been lobbying for them for some time.

"We are concerned that koalas will cross the road during the hazard reduction burn and in the following weeks adding to the toll."