Many of the trees were classified as "very high conservation significance", but are being felled to shave two minutes off the travel time between Ararat and Beaufort and make the rural highway safer.

The Western Highway is being duplicated from Ballarat to Stawell, at a cost of $662.3 million.

VicRoads estimated no more than 221 large old trees would be lost when it widened the 41-kilometre highway corridor between Beaufort and Ararat, in an environmental effects statement completed to gain project approval.

It said detailed road design would likely reduce the actual number of large old trees lost to less than 221.

But the roads authority has conceded it badly misjudged the amount of native flora it would have to remove, and now expects to cut down up to 885 large old-growth trees.

The figure is four times greater than what VicRoads predicted, because the authority's environmental effects statement did not count scattered trees that are not beside the road, but are being felled all the same.

VicRoads admitted its blunder following sustained pressure from activist group WHAM – Western Highway Alternative Mindsets.

WHAM spokeswoman Helen Lewers said the natural environment had been scarred by the careless destruction of hundreds of beautiful old trees.  

"When the 400-year-old Separation Tree was lost it made all of Melbourne very sad, so how come it doesn't matter if nearly 1000 like it are cut down?" she said.

She said work on the project should have ceased once VicRoads realised the number of trees to go far exceeded first estimates.

VicRoads chief executive John Merritt acknowledged the "discrepancy between the original estimate, which did not include scattered trees" and the true figure.

"Road safety is our utmost priority and roadside trees can be a hazard in run off road accidents," Mr Merritt said.

A more detailed planning report, from 2014, identified 1635 large old trees that would be affected, and VicRoads had saved about 500 of those, Mr Merritt said.

"The chosen route provides the least environmental impact and VicRoads will plant over 12,000 new trees and 50,000 other plants when major construction is completed," he said.

The error prompted an intervention from Luke Donnellan, the Minister for Roads, who wrote to Ms Lewers to inform her that VicRoads was revising its design for the next highway section to be widened, between Buangor and Ararat.

This would include "narrowing the median width where possible" and using concrete and wire rope safety barriers to minimise the construction area.

Victorian Greens leader Greg Barber said both major parties shared the blame, having exempted VicRoads from the need to gain permits for native vegetation clearing.

"When it comes to tree clearing, public authorities are some of the worst offenders, especially with the exemptions given to them back in the time of the Brumby government," he said.

"Victoria is the most ecologically damaged state in Australia and, in western Victoria, we have just a few shreds of remaining ecosystems, usually along roadsides.

“Every old tree is precious habitat and in this case VicRoads took the easy way out that was bad for the environment."

The Courier