While the backlog has dropped about 40 per cent since 2008, 131 heritage nominations are still waiting to be assessed, ACT Environment, Planning and Sustainable Development Directorate deputy director-general told a committee hearing on Monday.

However 38 of those nominations could be for the same place, Mr Rake said.

"One of the reasons items sit there for an extended period of time is complexity around a number of heritage values that could be argued for each particular place and that quite often we've received multiple nominations for the same place," he said.

"A number of the items that are on the list at the moment, there are 38 that we think are very probably duplicate nominations that cover such close ground they could sensibly be considered together but we need to work through a statutory process.

"Person A, person B will both identify a place with some commonality to the way they expressed the value claim for that place with some areas of difference so we need to work closely through the framework to assess that and make sure we're accurately assessing the interest as put to us."

His admission was in response to questions from Liberal politician Steve Dozpot, who had asked how it was possible nominations could sit in the heritage listing backlog for up to 16 years.

"Sixteen years, the pharoah probably built the pyramids quicker than that," Mr Dozpot quipped.

"In 16 years you think you could work out some of these issues in this time."

In January, Fairfax Media revealed the longstanding backlog was delaying the legal protection of some of Canberra's most historic sites, including Manuka Pool and the Kingston shops.

Planning minister Mick Gentleman told the committee the Heritage Council was receiving more nominations than before with a substantial amount of detail involved.

But while Mr Rake admitted the department only had the full-time-equivalent of 1.6 staff allocated to heritage assessments, he said it was not staffing problems holding them up.

"I think it's the Oaks Estate one you're referring to [that has taken 16 years], it's a very complex nomination in terms of the parties that are interested, the potential coverage of privately held land, publicly held land, public infrastructure, relationships to different elements of the history of the place, the heritage of Canberra and the region," Mr Rake said.

"The statutory test for including an item on our heritage register is that it meets a criteria to a high degree and sometimes we might be able to see that it's headed in the right direction across that threshold but we don't have enough information nor can we get it ourselves and so the heritage council has quite a task there to satisfy themselves and if we get it wrong, particularly in respect of private property, then we've potentially got another legal argument that's expensive."