On August 15, 1962, Indonesia and Netherland signed New York Agreement facilitated by the United Nations. According to the agreement, Dutch handed over West Irian to the United Nations. As the result, United Nations Temporary Executive Authority  (UNTEA) was formed.

The handover of  West Irian from the Netherlands to UNTEA officially run on October 1, 1962. Afterwards, on December 31, 1962, the Dutch flag in West Irian was replaced by Indonesian flag and the UN (UNTEA) flag alongside with it.

The UN Secretary-General came to Jakarta and Jayapura to clarify that the UN would guarantee the process of transferring UNTEA authority to the Indonesian Government in February 1963. Slowly, Dutch’s people and government left the land of West Irian. And on March 1, 1963, almost all positions in the UNTEA government were in the hand of Indonesia.

Three days later, May 4, 1963, Soekarno arrived at Kota Baru, Jayapura, West Irian which was previously named Hollandia. Soekarno’s speech:

“…. And, friends, what is it that is called Indonesia? What is
called Indonesia is the entire archipelago between Sabang and
Merauke. What is called Indonesia is what was formerly referred to
as the Dutch East Indies. What is meant by Indonesia is what the
Dutch called Nederlands Indië, that entire archipelago between
Sabang and Merauke which is composed of thousands of islands.
That is what is called Indonesia”

(Quoted from Soekarno’s Speech in Kota Baru, Jayapura, May 4th, 1963)

Furthermore, according to  the proclamation of The Republic of Indonesia which was held on August 17th, 1945, it can be inferred that Indonesia covered the territories colonized by Netherland. That is the entire archipelago between Sabang and Merauke. These evidences indicate that West Irian is already a part of Indonesia since then and even before the proclamation.