The backers of a multimillion-dollar highway linking Milford Sound with the West Coast are demanding a paper road vital to the project be returned to the maps.
The Hollyford road proposal hit a speedbump two years ago when backers discovered a paper road, which has been on official maps since the 1800s, had vanished.
The proposed link is a road from Haast to the Hollyford Valley in Southland. That would save motorists the current drive via Queenstown, which takes six hours.
It would be a 126km drive from Haast to Milford Sound, which would take less than two hours.
However, 98km of the project requires development of the paper road which vanished from maps some time after 1977.
Durham Havill is chair of the company behind the proposal, Haast Hollyford Highway Ltd, and said it had confirmed the paper road was never removed legally.
In response to an Official Information Act request, Land Information New Zealand (LINZ) admitted there were no records documenting correspondence about removing the road from the maps.
Mr Havill said that confirmed suspicions the removal of the route from maps may not have been done legally.
"They can't tell us anything about the road reserve, how it's gone from solid lines, to pecked lines...we can't find anywhere it was done legally," he said.
Given the lack of evidence of removal, the road should be returned to the books, Mr Havill said.
LINZ deputy surveyor general Anselm Haanen said the road's legality had always been questionable.
"Some maps published by the Lands and Survey Department during the 1970s and 1980s used pecked lines to depict the road, while others used solid lines, reflecting uncertainty of the road's status."
Kit Mouat, who worked as a district solicitor for lands and surveys for 23 years, said if a chief surveyor certified the road, a court process with public consultation was necessary to remove it from the maps.
Three past chief surveyors certified the road over the years, the first in 1880s.
Mr Mouat said in his time at the Lands and Surveys Department, the rule was once a road, always a road.
Another former Lands and Surveys employee, Milton Smith, is a credited supplier to LINZ.
He said establishing the existence of the paper road was just the first step towards the proposed road, and the question of who owned it should not be mixed with whether to develop it.
Mr Havill said while road advocates wished to avoid a costly court process, they were prepared to take legal action to reinstate the paper road and would be seeking legal costs.
That meant that, in the end, it would cost the taxpayer more money.