A former VicRoads senior adviser has slammed the roads agency for overlooking an alternative, cheaper route for the Western Highway duplication that would have saved more trees.

It comes amid concerns the cost of the highway upgrade has blown out by more than $40 million, well before a shovel has been put in the ground.

Hundreds of protesters are expected to join traditional owners, the Djab Wurrung people, at campsites dotted along the existing highway, in preparation for a standoff with police on Thursday.

Police are set to evict the traditional owners, who set up camp 14 months ago, to protect 700-year-old sacred Aboriginal trees from destruction to make way for the highway upgrade.