James Turrell's Skyspace has been among the gallery's most popular works since it was installed in 2010, and not just for its technical mastery and physical beauty.

The immersive work's location - tucked under a picturesque grassy knoll outside the gallery's front entrance - and the fact that it's open to the public 24 hours a day has made it Canberra's number one spot for lovers' trysts.

It's not hard to see why; the serene viewing chamber is both private and romantic, with its turquoise water, sloping walkway and soft ochre interior, and Turrell has thoughtfully rendered it the most dramatic place in Canberra to view sunrise and sunset.

But it's after dark that the place gets busy, with couples regularly meeting for some arty romance, spurred on, no doubt, by the fact that nothing inside can be seen from the outside, and that you can easily hear anyone approaching.

The gallery commissioned the work, aptly titled Within without, directly from the artist and unveiled it to coincide with the opening of the building's new wing in 2010.

Curator Simeran Maxwell says the work has been hugely popular ever since, and that the major retrospective of Turrell's work (currently showing inside) had only increased its success.

She says staff had been made aware almost immediately that the place was a popular lovers' spot, for the simple reason that Skyspace, being a priceless work of art, is fitted with security cameras.

These, she said, were there to prevent vandalism of the structure, but it was certainly the case that a lot more went on inside the space than mere star-gazing.

"There have been a few occasions where couples have taken things further," she said.

"The security guards usually just look away, because it's not hurting anyone."

She said Turrell himself was well aware of Skyspace's romantic allure, not least because several - extremely wealthy - couples in the United States have commissioned private Skyspaces to enjoy in their own, presumably massive, gardens.

James Turrell: A Retrospective is at the National Gallery of Australia until June 8.