Easing weather conditions have helped fire crews gain the upper hand over two bushfires still burning in New South Wales.

A bushfire near Alpine in the Southern Highlands and another near Lower Portland on the Hawkesbury River are both now listed as "advice", meaning they do not presently pose a threat to homes.

The Southern Highlands fire caused the Old Hume Highway and the railway line to be closed over night, but those thoroughfares have now been reopened.

However, trains are travelling slowly through the area and motorists are being warned of smoke in the area.

Rural Fire Services Commissioner Shane Fitzsimmons says crews are facing difficult conditions in the area.

"It's pretty rugged terrain, difficult to access, heavily forested country," he said.

"As we're seeing already today, a lot of those trees are damaged and they then become susceptible to collapse and fall along the fire ground, which is what we have got to be careful about when it comes to reopening roads and railway lines."

Commissioner Fitzsimmons says the fire near Hawkesbury is still being controlled.

"It was downgraded to advice around three or four o'clock this morning. There was more than 100 firefighters working on that last night," he said.

"It is going to require a deal of attention throughout today, but fortunately, like the other fire ground, conditions have eased considerably.

"Firefighters on the ground and the use of aircraft will seek to bring that fire under control throughout today.

"It's not declared contained yet, but it's not posing any threat to homes."

Commissioner Fitzsimmons says the next cycle of hot weather is expected to move through the state on Thursday, but until meaningful rainfall arrives the containment of the fire grounds will be tenuous for days, if not weeks.

Two fires still burning in state's south

Meanwhile, about 90 firefighters are working to contain two separate fires in remote forest escarpment in the state's south.

Both were ignited by lightning more than two weeks ago.

About 60 firefighters are trying to contain a forest fire burning through more than 7,000 hectares of Budawang National Park and private farmland around 25 kilometres north-east of Braidwood.

Specialist remote area fire crews are being winched into the region and water bombing is continuing today.

Without rain, the Shoalhaven Rural Fire Service says the fire is expected to continue burning for several weeks.

Further south, the Eurobodalla Rural Fire Service says 27 remote area fire fighters are trying to contain a 150-hectare blaze in the extremely steep and remote forests of Monga National Park, between the Kings Highway and Nelligen.

The fire has burnt out the historic Corn walking trail.