Australia's move to strip part of Tasmania's forest of its world heritage status one year after it was added is "very exceptional", the UN has told the Guardian, adding its experts could not recall such a case in recent years.

A Unesco spokeswoman said changes to world heritage sites were not uncommon. But, to the agency's knowledge, the appeal to remove an area so soon after a national government had asked for its addition was unprecedented.

"It is highly unusual that a boundary change entail removing an extension requested just one year earlier," she said. "Our experts in the world heritage centre have no memory of any similar cases in recent years."

Shifting political agendas within the deeply polarised community of foresters and environmentalists who live on its fringes have caused the Tasmanian world heritage area to be changed more often than any other Unesco site.

"This site's boundaries have already been modified four times, mainly for extensions. It's the only site on the world heritage list to have undergone so many revisions," the spokeswoman said.

The coalition government described the requested annexation of 74,000 hectares of forest as a "minor boundary modification", saying the forests were "degraded" – a claim quickly disputed by the Wilderness Society. The area is part of a 172,500-hectare addition to the world heritage area requested by Australia and approved by Unesco last year.

Unesco's world heritage committee will consider the amendment in June. It will decide whether the changes are classed as minor or major according to how they affect the overall value of the site. Minor boundary changes are significantly simpler to affect.

"The number of hectares is not what counts. Rather, changes are defined by their impact on what we call the "outstanding universal value" of the site; in other words, those characteristics that led to it being listed in the first place," the Unesco spokeswoman said.

Peter Valentine, associate professor at James Cook University, said the committee was unlikely to consider any revocation to be minor and raised concerns about the precedent it could set.

"Because Australia did make a claim that the extension was a valuable addition, supported by both IUCN and the world heritage centre, it is unlikely that the committee would agree to just excising part of the world heritage area at the wish of the new government. Such an action would be a serious problem for future world heritage protection everywhere.

"It is my view that what the Australian government is asking cannot be seen as a minor boundary change. If the area is now so badly damaged perhaps the committee would seek an explanation of how this happened and what steps would be taken to ensure rehabilitation (which is an obligation under the world heritage convention)."