The causeway on the Oxley Highway between Gunnedah and Tamworth has undergone $5 million of floodproofing works, which have been finished a year after originally expected.

The project was set back months after cracks were found in concrete culverts supplied by a manufacturer late last year.

Gunnedah Shire Council decided all the culverts should be replaced so there was a delay as new culverts were manufactured and frustrated drivers were forced to use a side-track while work was at a standstill.

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Tamworth MP Kevin Anderson said the flood improvement work near Carroll was "vital for the efficiency and productivity of heavy freight movements when the rains return".

"This section of roadway floods quickly and closes the highway, which is a problem for this very busy road," he said.

Mr Anderson said about 4000 vehicles travel the road each day and the highway could now remain open if it flooded.

Transport for NSW contracted the Gunnedah Shire Council to upgrade the road, "keeping the money within the community in a pretty tough time".