NSW Fire and Rescue were alerted to two grass fires burning on both sides of the highway, about 15 kilometres from Dubbo, around 10am.

Orana RFS team manager Lyndon Weiland said the two separate fires sparked interest with the RFS and police.

"It's strange for a fire to start on both sides of the road," he said.

"The Rural Fire Service is investigating the fire and assisting police with investigations."

NSW Fire and Rescue station office Mick Medlin said the cause of the fire is still unknown.

"Natural causes have been ruled out, it's now a matter of determining whether causes were accidental or deliberate," he said.

The fire on the eastern side of the highway was extinguished easily but the fire on the western side quickly spread to neighbouring properties.

Fire and Rescue requested immediate and heavy assistance from the RFS.

The Newell Highway was closed by Orana Highway Patrol in the early stages of the fire.

The eastern lane was re-opened once the blaze was controlled, with traffic assisted along the road via police escort.

Both lanes of traffic were opened at reduced speeds before returning to normal early in the afternoon.

It took 12 RFS tankers with 24 personnel and four NSW Fire and Rescue tankers with 16 personnel three and a half hours to contain the blaze.

RFS were patrolling burning hay bails late into the afternoon.

Superintendent Weiland said at its peak the fire had flames towering three metres high.

The status of the fire was constantly changing because of the winds.

The fire came to within a half a kilometre of property, but the property was well protected by tankers.

A number of hay bales, unharvested and harvested crop and fencing were destroyed in the incident.

An onlooker was distressed as he watched the fire edge close to his property.

He said he could not believe how fast the fire had travelled.

Superintendent Weiland estimated damages to total between $10,000 and $20,000.

He said this is a timely reminder for residents to be "very, very careful with their actions."

Station Officer Medlin said members of the Orana RFS should be applauded for their efforts.

"The RFS volunteers managed to leave their jobs and houses quickly and got the fire under control easily," he said.

Residents are encouraged to make a plan if they live in fire-prone areas, Superintendent Medlin said.

"Fires can happen at anytime- residents need to be prepared," he said.

1 PM: The Newell Highway is now open. Fire crews are still working in the area. 

12 PM: Alternative traffic conditions are now in place. Motorists are able to travel via police escort. 

Allow additional travel time. 

11.30 AM: Both the NSW Rural Fire Service and Fire and Rescue NSW are on scene. 

10.45 AM: The Newell Highway has been closed in both directions due to a grass fire 15km north of Dubbo. 

The highway was closed about 10.30am on on Tuesday morning. 

Light vehicles are being diverted via Mendooran Road and the Castlereagh Highway while heavy vehicles are being diverted via the Mitchell Highway, Trangie Collie Road and the Oxley Highway. 

Emergency services are on scene and the fire is being controlled. 

There is no forecast as to when the highway will reopen. 

Motorists should allow additional travel time or delay their journey. 

More to come.