At present motorists heading north from Kiama can pass under the Oak Flats interchange and continue along the highway to Albion Park Rail.

After the construction of the bypass is complete, that will no longer be possible as the traffic will travel left behind Albion Park.

Instead motorists wanting to stay on the highway will need to travel up to the interchange and then exit back onto the highway.

"To accommodate this, the interchange will feature traffic signals designed to cater for traffic movements as they change throughout the day and into the future," a Roads and Maritime Services spokeswoman said.

"The installation of the traffic signals will mean the interchange operates much more efficiently than it does currently and will improve safety by minimising traffic conflicts."

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The bypass itself will travel under the interchange, which the spokeswoman said will not require any structural alterations.

"The Oak Flats interchange was designed to allow a future roadway to pass underneath," the spokeswoman said.

"The Albion Park Rail bypass design factors this in and construction will not affect the structural integrity of the interchange.

"Clearing work has been carried out to allow for construction of the bypass and only minimal excavation will be required under the interchange itself for the purpose of installing road surface."

The $630 million four-lane Albion Park Rail bypass is expected to be completed by 2022.