If the Dart Passage Tunnel, proposed by Milford Dart Ltd, or the Fiordland Link Experience, proposed by Riverstone Holdings Ltd, gain approval, Unesco may send a monitoring group to New Zealand to assess the impacts of the developments.
This could lead to the Te Wahipounamu heritage site being deleted from the World Heritage list. It could be added to the List of World Heritage in Danger.
Unesco public relations division media relations chief Sue Williams said yesterday the organisation became involved after receiving "a number of reports by third parties" earlier this year.
It contacted the New Zealand authorities, requesting information on both proposals, "including their legal status and stage of implementation", Ms Williams said when contacted at her base in France.
The Department of Conservation confirmed the two proposals had been "approved in principle" and provided Unesco with copies of impact studies and proposed mitigation measures.
Unesco's World Heritage Centre and the International Union for the Conservation of Nature, an international environmental organisation, were assessing the information.
The next step could be the preparation of a report to be considered at the next World Heritage Committee session, in June or July next year.
From that meeting, officials could be asked to visit the area to assess the impact of the proposals.
"The World Heritage Centre has requested the New Zealand authorities to keep it informed of any development, including the outcome of the public hearings," Ms Williams said.
Late last year, Conservation Minister Kate Wilkinson announced her intention to grant concession applications to both companies. Submissions were then called for and were heard over several weeks this year by a Doc hearings panel in Queenstown and Te Anau.
Doc media adviser Reuben Williams said yesterday Unesco had no legal jurisdiction over the concession applications process.
Unesco was interested in world heritage but "they won't be in a position to be involved ... in the decision-making".
Doc was still preparing reports on the applications to be forwarded to the delegated decision-maker, Doc operations deputy director Sue Cosford, he said.
"No formal decisions have been made ... it could be some time off."
Milford Dart Ltd director Michael Sleigh said he "just can't see how" the 11.3km tunnel could jeopardise the World Heritage status. It would have a "minimal impact" compared with other developments at Milford Sound.
"If they [Unesco] were coming, they should be having a very close look at what the various Te Anau-based stakeholder groups [in] the Milford village [have done] in terms of the 6ha of native forest they removed to allow for commercial expansion.
"It's far more dramatic than anything we're proposing.
"They should also be talking to the people at Gunn's Camp [in the Hollyford Valley] and the mayor about recent support for the Haast-Hollyford highway and what impact that will have on the World Heritage status."
Riverstone Holdings Ltd director John Beattie said Unesco would be "absolutely satisfied" with his company's project, which would be an "exemplar for future activities inside the Department of Conservation estate in New Zealand".
He likened the project to the 7.4km Skyrail Rainforest Cableway, near Cairns, in northern Queensland.
"I'm advised by the operators ... that the benefit, they believe, of the infrastructure to the World Heritage status position - the knowledge and understanding of the rainforest - has been considerably improved ... as a result of the infrastructure being in place.
"We expect the monorail will be no different in that regard and we'd expect, having been through a detailed eight-year process, which has been extremely robust, with Doc ... that Unesco will respect the integrity of the Doc process and this will cease to be an issue."
The proposals
Dart Passage Tunnel: An 11.6km commercial bus tunnel from the Routeburn road in Mt Aspiring National Park to the Hollyford road in Fiordland National Park. Cost: $150 million.
Fiordland Link Experience: A catamaran trip across Lake Wakatipu to Mt Nicholas, an all-terrain vehicle trip to the Kiwi Burn, then a 43km monorail journey to Te Anau Downs, on Lake Te Anau. Cost: $175 million.
World Heritage sites
• 962 sites worldwide, including three in New Zealand.
• Te Wahipounamu ("Place of Greenstone") covers the Aoraki-Mt Cook, Fiordland, Mt Aspiring and Westland National Parks; included in 1990.
• Tongariro National Park included in 1990, New Zealand Subantarctic Islands in 1998.
• To be included, sites must be of "outstanding universal value".
