The Australian government has completed and opened the Frederickton Interchange on the $618m Kempsey Bypass in New South Wales.

The interchange will be used to divert highway traffic in both the directions for about 1km before reconnecting with the existing highway.

The opening of the interchange is a major step towards the completion of the fully-federally funded Kempsey bypass, which in turn is important for the duplication of the Pacific Highway.

Duty Senator for Cowper Matt Thistlethwaite said the milestone in the delivery of the new bypass will allow the project team to complete their work on the northern side of the Macleay River.

"Once completed, the new bypass will take up to 2,000 trucks a day off the local streets of Kempsey and Frederickton and deliver faster, safer and smoother driving conditions for the hundreds of thousands of motorists who use this stretch of highway each year," Thistlethwaite added.

"At 3.2 kilometres in length, the new bridge being erected over the Macleay River and floodplain will be Australia’s longest."

NSW Member for Port Macquarie Leslie Williams said the Kempsey Bypass will be a 14.5km four-lane divided highway to the east of Kempsey and will involve around 1.4 million cubic metres of earthworks.

"The interchange will allow for completion of the Frederickton levee, which will improve flood immunity for the local community to a 1 in 100 year flood level," Williams added.

"The community will also benefit from a range of new flood mitigation measures such as house raising, stock mounds and flood compensatory work."

The project is creating many direct and indirect jobs and is expected to contribute towards the economic growth of the region and local businesses, the government said.