Update: The Hume Highway has been closed in the Southern Highlands.

The Hume Highway is now closed in both directions due to a bushfire. It is closed between Highland Way at Marulan and the Illawarra Highway at Sutton Forest.

Motorists are advised to avoid the area and continue to delay all non-essential travel.

On the South Coast, the Princes Highway is closed between:

Motorists should stay away from these areas as there are no diversions available. Anyone already in a town is advised to stay in their location as there is no forecast for when the highway will reopen.

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The Green Wattle Creek fire in the Southern Highlands has been upgraded to watch and act level on Tuesday as firefighters prepare for a southerly change in the afternoon.

The fire remains active in several areas and there is an increase in activity in the Wombeyan Caves Road area, north west of Mittagong.

If you are in the area of Wombeyan Caves and Mittagong, monitor conditions.

RFS Southern Highlands said on Facebook at 2pm that "activity on the southern extent of the Green Wattle Creek fire was increasing".

"North west winds were continuing to impact the fire ground. A reminder for those to the south and south east of the fire to enact your Bush Fire Survival Plans," the post said.

"Areas West of the Wollondilly River along Wombeyan Caves Rd in the vicinity of Tallygang Mountain Road should seek advice from fire fighters on the ground. It may be to late to leave."

Residents in the Yanderra, Hill Top, Buxton, Yerrinbool and Colo Vale are advised to monitor conditions and be alert for embers and spot fires.

Firefighters, supported by water bombing aircraft, are working to strengthen containment lines across the fireground.

The blaze is more than 227,000 hectare blaze.

There are technically issues with updating the Fires Near Me app.

There are no evacuation orders or centres at this stage however residents can go to Mittagong RSL or Moss Vale Services Club to escape the heat, and can stay with animals at Bong Bong Racecourse and Moss Vale Showground.

Southern Highlands RFS public liaison officer Elizabeth Ellis said there was very little fire activity on the eastern side, near the Hume Highway, as of Tuesday morning.

Read more: South Coast fires at emergency warning level, highways closed

"There are no areas or townships that are imminently being threatened," she said.

"There is a warning that the Hume Motorway may close at short notice."

Fire crews are protecting rural properties on south west side of the fire front in the Wingecarribbee shire whilst other firefighters are patrolling, staged and ready to move if required.

The southerly change is expected about 5pm on Tuesday.

Ms Ellis reminded people near the fire to be prepared with their bags packed in case fire activity escalates.

"People need to think where they will go and how they will get there," she said.

"Think of alternative routes. Take out the Christmas card list and think about who you can go stay with for a New Year visit.

"People need to monitor the situation and be ready to go if required."

The Southern Highlands train line is partially closed.

Buses replace trains between Moss Vale and Campbelltown due to the severe weather forecast and advice of the NSW Rural Fire Service.

Allow extra time and listen to announcements.

Residents are reminded to avoid non-essential travel on Tuesday due to the weather forecast and fire danger rating.

The Princes Highway is now closed between Jerrawangala and Milton due to a bushfire and traffic is already very heavy northbound on the Princes Highway in Ulladulla and is queued back to around Lake Tabourie.

The Princes Highway is also closed between Batemans Bay and Moruya, between Cobargo and Brogo, and further south between Genoa and Cann River (Victoria), and in Narrabarba between Imlay Road and the Victorian border.

Keep up to date on fire situation through the RFS website and Fires Near Me app.