UPDATE 5.50PM: The Maryborough-Hervey Bay Rd has reopened, more than nine hours after the four-vehicle crash which closed it on Tuesday morning.
A police spokesman said a diesel fuel spill has been cleared and the road is again open to traffic in both directions.
UPDATE 5.11PM: A police spokesman said the Maryborough-Hervey Bay Rd will remain closed until 6pm due to an excessive amount of diesel fuel on the road.
Traffic is currently being diverted via Torbanlea.
The police spokesman also said the 21-year-old woman airlifted to the Royal Brisbane Hospital is in a stable condition.
UPDATE 4.10PM: The Maryborough-Hervey Bay Rd is still closed at Dundathu as workers attempt to tow the semitrailer involved in the four-vehicle crash out of the bush.
An AGL Action Rescue Helicopter spokeswoman said the truck ploughed through several metres of mud and bushland before coming to rest.
She said the 21-year-old woman, who was driving the car which hit the semitrailer head-on, was critically injured and flown to Brisbane.
The spokeswoman said the flight doctor and intensive care flight paramedic treated the woman for multiple trauma injuries to her head and limbs.
A police spokesman did not have an estimate of when the road might be reopened.
UPDATE 11.15AM: Police have confirmed a vehicle may have crossed to the wrong side of the road causing the four-vehicle crash on Maryborough-Hervey Bay Rd at Dundathu about 8.30am on Tuesday.
"Early indications would tell us that a vehicle may have crossed onto the incorrect side of the roadway causing the accident," Sergeant Wayne Tincknell said at the crash scene.
"The driver of one of the vehicles, a young lady, has been airlifted to Brisbane, the Royal Brisbane Hospital, in a serious condition."
He said only one person was injured in the crash and the driver of the semitrailer and other vehicles involved had been "very lucky".
"I'd just like to remind people to be careful whilst driving on the roads," Sgt Tincknell said.
"We've got a long weekend coming up and accidents do happen."
The road is still closed and a Maryborough police spokesman said he did not expect it would reopen before 4pm.
UPDATE 9.45AM: A woman has been freed from a hatchback and transferred to a waiting helicopter after a four-vehicle crash on Maryborough-Hervey Bay Rd.
Maryborough Towing's Chris Sawyer, who is at the scene, said a Richers Transport semitrailer was off in the bush and two other cars were in the gutter.
He said the crash happened at the overtaking lanes at Dundathu and one of the drivers told him she saw a car on the wrong side of the road just before the crash.
Mr Sawyer said the semitrailer hit the black hatchback, believed to be heading towards Maryborough, head-on.
He said the drivers of three of the vehicles were walking around.
UPDATE 9.25AM: Firefighters have cut the roof off a black hatchback involved in a four-vehicle crash that has closed the Maryborough-Hervey Bay Rd.
Firefighters are working to free a woman from the vehicle while ambulance officers hold up a drip on the driver's side of the car.
It's believed the AGL Action Rescue Helicopter has been called in.
EARLIER: A four-vehicle crash on the Maryborough-Hervey Bay Rd between Susan River and Dundathu has resulted in the closure of the road.
Police said the road was likely to remain closed for hours and are advising motorists to avoid the area and travel via Torbanlea.
The crash, about 1km from the Prawle Rd intersection on the Hervey Bay side, is believed to have involved a truck and scattered debris on the road.
No further details are available at this stage.