The first thing to note about the National Gallery of Australia is that the enjoyment stems from both the artworks inside the building and those outside. The permanent collection of Australian and international art is more than adequate reason for making the visit (Boyd, Nolan, Roberts, Streeton, Brancusi, Hockney, Pollack to name just a few), not to mention the best collection of Aboriginal art in Australia, but it is the outdoor artworks which really set the NGA apart from many art museums. First of all, see the sculpture garden between the gallery and the lake where outstanding sculptures are featured amongst trees and shrubs. There is also the wonderful Skyspace installation by James Turrell near the front entrance. It is a beautiful blend of architecture and astronomy. This is a feature best seen at dawn or dusk to appreciate the interplay between light and building.
Because entry is free for the permanent collection, one can go in and out of the building to rest in the sculpture garden as well as taking in the artworks there. However, to fully appreciate the sculpture garden be there at midday to see the beautiful, ethereal fog sculpture which adds so much beauty to the adjoining reed pond.
My recommendation for visitors who can spare a full afternoon is to arrive at midday to see the fog sculpture in the sculpture garden, have lunch, enjoy the gallery permanent collection, visit the National Portrait Gallery opposite the NGA and finally, return to view the Skyspace at dusk (even at night if moonlit).