Although the size of the gift may have been overwhelming, its real importance lay in its quality. This was the artist's private collection, on many occasions works which he had done because he felt he needed to do them, rather than what was called in the family the "gilded chestnuts" made for exhibitions and sales. In other words, this was the private Arthur Boyd, which has remained little known and rarely exhibited.

Ten years after the gallery received the gift, in 1985, Grazia Gunn mounted her Arthur Boyd, seven persistent images, a pioneering exhibition in which the curator traced the obsessive repetitions in Boyd's iconography, on occasion illuminated by the artist's own comments on this art practice. This new exhibition, curated by Deborah Hart, re-examines this material, supplements it with a number of other key pieces which the gallery has acquired over the years, and in about 180 items presents another facet and reading to the complex artistic genius of Boyd. This is the most significant Boyd exhibition to be mounted since the artist's death in 1999.

No matter how well you think you know the art of Boyd, there is work in this exhibition which you have never seen before. The early "agoraphobic surrealism" of the Murrumbeena and South Melbourne years during the Great Depression and the war years is generally well known to art audiences. However, the surviving fragment of the Grange murals, which the artist executed in 1948-49 for the house owned by his uncle, the wonderful novelist Martin Boyd, until now has been known only in black and white photographs. In an accomplished classical style of the Old Masters, Boyd demonstrates his versatility and mastery of the language of the Biblical grand narrative, but he delivers it with an unmistakably Australian accent. The idea that scenes of Biblical significance could be tucked away in the scrubby Australian bush was to remain with Boyd for the rest of his life.

In the 1950s, prior to his departure for Europe with his family in November 1959, Boyd enjoyed a degree of commercial success as Chagallian- inspired romantic forms edged out the tortured forms of earlier years which drew on the heritage of his father Merric Boyd's superb eccentric drawings and the Dostoevskian atmosphere of Murrumbeena. In this exhibition we encounter a wealth of ceramics, including a superb maquette for the Olympic sculpture for Melbourne, and the Bride painting from the Alan Boxer collection. The Half-caste bride series was to become the artist's most successful early body of work and was shown to considerable critical acclaim on his arrival in London. If you want to see a really strong series of Bride paintings you need to go to Melbourne to the Heide Museum of Modern Art which has an exhibition dedicated to the series opening in November.

The main strength of the Canberra exhibition is Boyd's London work, his work on Robert Helpmann's Electra ballet, the powerful Nebuchadnezzar paintings and work on the St Francis series and their resolution as a brilliant series of tapestries hung as an installation in the final room of the exhibition. Boyd was actively involved in the Nuclear Disarmament movement and protested at Australia's involvement in the civil war in Vietnam. Frequently he was obsessed with questions of ethics and the whole idea of the impotence of an artist surviving in such an ugly world. He opposed nationalist narrow-minded chauvinism and militarism, he was not a player for "team Australia", but a committed humanist who ascribed to the "team world" philosophy.

The greatest revelation in this exhibition is the Caged painter series which is allowed to unfold in all of its monumental splendour. The great canvas, Paintings in the studio: 'Figure supporting back legs' and 'Interior with black rabbit', (1973-74), measuring more than three by four metres, is well known and stands up well in any international company. Many of the other paintings in the series are far less well known and have rarely been seen. Boyd, after a decade in England, returned to Australia in 1971 and painted in the country around Canberra. On returning to Britain he worked on the Caged painter series and refusing to break up the series, he donated the whole lot to the Canberra gallery.

Much of Boyd's work suffers through over interpretation, and one can delineate the imagery of the tormented artist held by his legs by his Muse, while with one hand, like Judas, he clutches a bag of gold by selling out his talent to the market and for which his brushes in his other hand are now upside down and with which he is unable to work. Like so much of Boyd's art, the Caged painter series operates on allegorical and anagogical levels denoting a tortured frustration, like a writer's block, and yet insistent that painting was relevant and alive despite the art schools of the day declaring it dead. As a child, I remember Boyd telling me that why he loved Dostoevsky was because he dared to say "beauty will save the world", despite being ridiculed and declared mad by his contemporaries. For me, on a rather basic level, the Caged painter series screams out that indeed beauty will save the world, because nothing else will.

Many artists age badly. Some "burn out" by the time they are 40 and their art becomes a hollow repetition of juvenile rages for the rest of their lives. Boyd's oeuvre was uneven, but with time it became stronger. He knew what was his best work and that is why he gifted it to the nation. For him, an artist had to adopt an ethical stance, no matter how unpopular and against the flow of fashion. By being a little unfashionable in his own day, Boyd has seized immortality.

This is a great exhibition because the art is of a very high order and it tackles some of the great themes of being, including what it means to be human, the role of the creative individual in a world of nightmarish materialism and the belief in the redemptive power of beauty. Unlike so many artists before him, stubbornly, Arthur Boyd has not left the building.

Arthur Boyd: Agony and Ecstasy at the National Gallery of Australia closes November 9.