Gunnedah Shire Council will carry out the the $8.2 million upgrade of Grain Valley Road.
The council originally planned to go to tender this month but decided to keep the project in-house.
"It was determined that there are likely to be many variables onsite, most notably, existing depth of gravel, therefore council are best placed to manage the project and will deliver the works," council's chief engineer Dan Noble said.
Mr Noble said the detailed design works were on schedule but had presented some challenges.
"Given the variable gravel depth, there will be at least three different pavement designs constructed," he said.
"The biggest challenge at present is the availability of water for the pavement construction given the ongoing dry conditions."
The engineer said the project was still on track for construction to start in December.
Mullaley residents are eager to see the remaining 17.4km of the notorious unsealed road upgraded after state government funding for the project was announced in November 2018.