The Cooks, which had been ruled by New Zealand from 1901, in 1965 became self governing in free association with New Zealand.
Mr Key says, at that time, both countries entered into a unique bilateral relationship.
He says New Zealand welcomes the opportunity to celebrate that and to reflect on an important partnership based on shared values, history, and the positive contribution Cook Islanders make to New Zealand.
Mr Key will be accompanied by the Pacific Peoples Minister Peseta Sam Lotu-Iiga, the Associate Minister of Foreign Affairs Todd McClay, Maori Development Minister Te Ururoa Flavell and the Internal Affairs Minister Peter Dunne.
The delegation will also include the Opposition Leader Andrew Little, the New Zealand First Leader Winston Peters, and a number of other MPs.