More than 125 native trees have been cleared from a 70km stretch of State Highway 6 between Haast Pass and Haast township by Transit New Zealand.
The national roading authority is half way through a $180,000 tree-felling operation on the Transit-managed road reserve, which is expected to last another two weeks.
By the time the operation is completed, 250 native rimu, beech and matai trees will have been chopped down in the area, which runs alongside a World Heritage protection site where any felling of native trees is prohibited.
World Heritage sites are nominated by Unesco and picked because they represent the best examples of the world's natural and cultural heritage.
Transit southern zone operations manager Peter Connors said the ageing native trees posed an "untenable" risk and needed to be chopped down to protect motorists.
The SH6 road reserve was not in the World Heritage site and the trees could be cut down because they were on the Transit-managed road corridor reserve.
Falling trees posed a great risk in the mountainous Haast Valley, which was prone to significant storm events, slips and flooding.
Transit was not in the business of chopping down trees, but the 250 in question had grown to a stage where the risk they posed needed to be managed, he said.
Transit was not required to consult any other party, except the Department of Conservation which had stewardship of the Haast World Heritage site.
The authority did not seek to consult any other organisations, nor the local communities of Haast and Makarora, he said.
"At the end of the day, when a tree falls on to a bus and kills someone, it will be us who has to stand up and defend a decision as to why we didn't take action," Mr Connors said.
Transit's primary job was to maintain the safety of the national highways for all road users, and felling trees was part of the authority's legitimate risk mitigation.
Doc had been involved with "individual investigations" into each tree, with 10 subsequently removed from the culling list.
Where possible, trees were cut so they fell to and remained on the forest floor, Mr Connors said.
