The group pushing for a Haast-Hollyford road is adamant it stacks up economically, but Forest and Bird says its sums show it cannot possibly make a good return for investors.
If approved, the 127km toll road would slice through the World Heritage Area, connecting the road ends at the Cascade River with the Hollyford Valley, north of Milford Sound.
A large part of the proposed route south of Jackson Bay follows the exact line of the volatile Alpine Fault.
A private, but unnamed, overseas investor has offered up to $220 million to finance it, if the road's promoter, Westland District Property Ltd, a division of the Westland District Council, can get resource consent. It expects to recoup its investment with toll charges.
However, in 2010, Christchurch businessman and Scenic Circle Hotel chain founder Earl Hagaman commissioned a report suggesting it would cost from $225m and $315m. Forest and Bird advocacy manager Kevin Hackwell has been doing his sums, and said this week the area was one of the most wet, rugged and geologically active in the country.
He noted that the unnamed financial backer was not from New Zealand.
Using government transport figures, Mr Hackwell based his calculations on when the South Island roads were busiest. He considered the 195,000 vehicles that drove south from Haast, and added that to the 282,000 from Wanaka going the other way, then added in the 220,000 vehicles that currently go through the Homer Tunnel.
Even if every single one of those vehicles used the new toll road, that would produce a return of about $14m a year; which is only a 5 per cent return on the investment for building the road, with a bit set aside for maintenance.
But Westland District Property says Forest and Bird has made a key mistake by assuming they would charge a $20 toll per car. In fact, it would be $20 per person; including everyone on a tour bus.
The property company calculates that the average number of people per car is 2.8, which would provide a far healthier return than Forest and Bird thinks.
Mr Havill said his company's figures had been audited by three parties; its own consultants, the offshore partners, and the New Zealand partner, which had signed a memorandum of understanding to design, build, finance and maintain the road, and collect the toll for 30 years.
All three projected return figures were "within 3-4 per cent of each other".
Mr Havill said the design company had estimated the road at $205m and Westland District Property placed $15m on top for a contingency.
The earlier Hagaman proposal, following the longer coastal route, technically more difficult and with environmental impacts the property trust considered unacceptable, he said.
However, the new inland route will require only two major bridges across the Cascade and Pike rivers, plus a number of smaller bridges. It would need 97km of new road, and 35km of existing road would need to be upgraded.
Mr Havill said the design-build partner was aware of the Alpine Fault risk, but that risk was minimised because the country was not steep.
It would take three to four years to build.
