Update at 3.30pm: Southbound lanes continue to be closed and diversions are in place.
Southbound to exit the highway at South Gundagai via Middle Street, Mount Street, Gocup Road, Snowy Mountains Highway and return to the Hume Highway.
The diversions are suitable for all vehicles.
Police Crash Investigation Unit, Transport for NSW and emergency services remain at the scene.
Earlier: A two-vehicle crash on the Hume Highway at Tumblong has closed both southbound lanes.
Emergency services responded to the scene after reports of the crash about 1.30pm today.
A NSW Fire and Rescue spokesperson said it appeared one of the vehicles involved was a 4WD with a trailer.
"I can confirm that Fire and Rescue, the rural fire service, ambulance and a helicopter is inbound to help with the rescue," the spokesperson said.
"The road will be shut for some time as there'll be some rescue work."
NSW Police will provide an update shortly while NSW Ambulance have been contacted.
Live Traffic reports that motorists are getting past in the shoulder.
It advises motorists to allow extra time and exercise caution.
In a social media statement, Gundagai SES confirmed it has also responded to the incident.
More to come.