The Super Star rose is a colourful climbing plant with salmon-pink-orange flowers. It was first bred in Germany in the 1960s but is apparently very popular worldwide these days. I can’t tell you why it was chosen to be one of a themed set of roses stamps in New Zealand in 1975, though I can tell you that the set were definitives (a term familiar to regular readers of this site) and that roses are both popular and productive in NZ.
The term superstar originated in the 1830s in a book about cricket and was used initially to describe sporting stars. It was popularised in the 1960s by Andy Warhol.