The NSW Rural Fire Service says the Blue Mountains fires will occupy firefighters for weeks, even once they have been contained.

Spokesman Joel Kursawe has told ABC News Online that today is crucial in determining whether the fire stays behind containment lines or breaches containment and burns towards western Sydney.

“We’ve been working very hard for the last couple of days to get back-burns in and try and work towards containing the fire,” Mr Kursawe said.

“But if these conditions get to what they’re forecasted today – 90km/h winds in some areas – we could see this breach the area it is in now and creep down into the backend of Sydney, in through Richmond, and Colo and Kurrajong and places like that up in the Hawkesbury.

“And once it’s there it’s then got the potential to keep running.

“Again, that’s dependent on the wind direction as well.”

Mr Kursawe says if authorities are unable to get on top of the NSW bushfires with back-burning and crews on the ground, it “has the potential to burn for weeks”.

It may be that firefighters will be successful in containing the blaze, but Mr Kursawe says that does not mean fire crews can pack up and go home.

“Regardless of how far it travels today, we are likely to see this fire continue to go still for coming days into next week,” he said.

“Even if we get containment on the fire, there’s still going to be a lot of active fire within the burnt areas and that’s going to require the crews to patrol this fire for weeks once it’s out.”

Rain can be a ‘hindrance’

A storm cell brought small amounts of rain to the Blue Mountains fires yesterday, but it did not have a big effect.

Mr Kursawe says days of consistent rain would help firefighters with a blaze as big as in the Blue Mountains, but small amounts can actually be a hindrance.

“We would want to see a couple of days of good solid rain, but even anywhere up to 20 or 30 millilitres would be a blessing in disguise for the guys on the ground, as opposed to the 2 (millimetres) to 5mm that they saw overnight,” he said.

He says light rain can slow or halt crucial back-burning operations depending how heavy the drizzle is.

“It’s a double-edged sword,” he said.

“Rain is good for fires, but minimal amounts like we saw overnight prove to be more of a hindrance and hamper back-burning efforts, which we’ve also been racing against to clock to try and complete ahead of the weather that we’re seeing today.”