The milestone was reached late on Wednesday, RFS Southern Highlands community safety officer, David Stimson said.

"Since the November 29 declaration, it has pushed everyone to the limits," he told The Post.

"It's been a very destructive fire and it's been hard to contain. It's taken two months, it's been very large and it's burnt an area never burnt before (in national park)."

The blaze has torn through 278,722 hectares in the Southern Highlands and around Wombeyan Caves, near Taralga. Just before Christmas, it claimed two firefighters' lives when their vehicle rolled at Buxton after a tree hit it.

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In this area, the Taralga community has been on tenterhooks as the fire broke out around Wombeyan Caves and spread to Mares Forest. The town has been used as a staging post and has been abuzz with activity.

Inspector Stimson said every strategy was thrown at the blaze to achieve containment.

"At the moment we're experiencing conditions we've never seen before. The ongoing drought and lack of moisture in the soil has helped create this behaviour. As I and the Commissioner (Shane Fitzsimmons) have said, these fires are unstoppable and very hard to contain."

He speculated in 'normal times' it would take half the time to extinguish.

On Thursday, crews were on the ground still looking for hotspots, aided by aerial infrared cameras. Inspector Stimson said he hoped the fire could be extinguished by next week.

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He was also confident that containment work undertaken in the past few weeks would hold in the hot days ahead. At the same time, he acknowledged the 39 and 40-degree temperatures would pose some challenges.

Upper Lachlan Shire Mayor John Stafford praised firefighters' and the community's efforts.

"Full credit to the people have worked so hard," he said.

"The local crews have been fantastic and have been going since early December. We've had crews from everywhere here and it's been pretty awesome."

But the Mayor said it had also been a difficult time for business with Wombeyan Caves' closure as a result of the blaze, and the loss of through traffic. In addition, people in the town weren't moving about as much due to fire and smoke.

Cafes, hotels and his own business, Taralga Wildlife Park had been affected.

"Tourism across NSW is down and this community is no exception," Cr Stafford said.

"...It's been a pretty bleak Christmas across the board for so many people. Our hearts go out to people on the south coast and other fire-hit areas. We haven't suffered like them but it has still impacted."

Like the local brigade, he was hopeful containment lines would hold in the weekend's high heat but urged residents to remain cautious.

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Meantime, the Morton fire around Tallong and the Southern Highlands is being controlled.

Inspector Stimson said backburning was still being undertaken and there were some aerial bucketing exercises in the area on Thursday.

Residents could expect to see more smoke but he was confident that strategies would soon achieve containment.

However he urged people to remain vigilant.

On Thursday, Southern Highlands RFS posted a Facebook message stating that contrary to reports, there had been no order to Tallong residents to evacuate.