Under the public areas more than 2000 artworks are stored in carefully controlled conditions at any one time. On Sunday, which is International Museum Day, just 40 people will get a chance to see where they are kept.

National Portrait Gallery registrar Bruce Howlett, who has been curating the collection for the past 10 years, said visitors were always fascinated to see how Australia's artworks were cared for and maintained.

"They usually just see the final product, the works of art on the wall, rather than all the activities that go on behind the scenes to put the works on display," he said.

"Down here there are three main storage areas ... we'll show the visitors the rooms and then we'll describe the activities and what takes place in them."

Each of the three rooms houses a different variety of artwork – one for smaller drawings and prints, one for sculptures and a final one for paintings – all kept at a precise temperature and humidity to avoid damage.

As part of the tour visitors would be able to see newly acquired historic photographs and a commissioned portrait of Midnight Oil and the process by which is was created, Mr Howlett said.

"[Visitors] are first and foremost interested in being in a place that they normally wouldn't have the opportunity to see, and see parts of the collection that aren't usually on display," he said.

The tour of the gallery's backrooms had been an annual event for a number of years and always sold out quickly, a gallery spokeswoman said.

"The portrait gallery is home to the nation's portrait collection, through which we tell extraordinary stories of encounter, exploration, independence, individuality and achievement in Australia," she said.

"Visitors to the gallery can follow the development of portraiture from oil painting to digital media."