The watercolour by former pupil and arts master, North Otago artist Colin Wheeler, is a detailed depiction of one of the school's dances in the 1950s.
Painted by Wheeler in 1953-54, it is a major departure from the style with which he is commonly associated - oil paintings depicting North Otago, farms and landscapes and, more recently, composites of Oamaru scenes.
Gallery director Warwick Smith said the picture was full of people enjoying the dance.
The painting was remarkably detailed, and showed some well-known staff and pupils, although they may not wholly recognise themselves: Wheeler painted some faces on different bodies, and one master's face appears on three people.
"It's a truly epic painting, an extraordinary water colour full of people and little episodes," Mr Smith said.
He has enjoyed closely studying it to recognise people he knows.
The painting has never been exhibited before.
It used to hang unframed outside Wheeler's Oamaru studio and is now owned by a private collector.
The exhibition, which officially opens tonight and runs until August 31, is called Songs May be Sung, after a line from the school song.
It features about 30 works by past and present pupils, including some of New Zealand's renowned artists - Wheeler, Colin McCahon, Bing Dawe, Peter Cleverley, John and Ewan McDougall, Burns Pollock.
Works include paintings, sculpture, prints and jewellery.
