The Department of Conservation has been told a proposed bus tunnel to Milford Sound would threaten the area's world heritage status.
The claim was made at the first day of hearings in Te Anau Monday on an 11km tunnel under parts of the Mount Aspiring and Fiordland National Parks.
The planned tunnel aims to make the journey from Queenstown quicker.
DoC says it has received more than 1200 submissions with 859 opposed to the plan and 400 in support.
Venture Southland's strategic projects manager Steve Canny told the DOC panel Fiordland was recognised as a World Heritage area in 1990, but it would be delisted if major developments like the tunnel go ahead in the national parks.
Mr Canny says when Germany's Elbe Valley was delisted tourists numbers dropped dramatically.
Milford Dart Ltd is applying for a concession from DoC to build the tunnel.
The plan also includes 150 metres of road entry ways in the parks at either end of the bus-only tunnel.
Eight days of hearings began on Monday with submissions from representatives of the Southland District Council and Venture Southland.
The authorities are arguing the plan will give private rights over parts of the national parks and cause noise pollution, especially during construction.