Red and black have long been traditional colors of many Papua New Guinean tribes. Black-white-red was the color of the German Empire's flag, which had colonised New Guinea prior to 1918. The bird of paradise is also found on the national coat-of-arms.
Prior to 1970, Papua New Guinea was known as the Australian Trust Territory of Papua and New Guinea, and its flag for sporting events was the bird of paradise on a green background. In 1970 the Australian Administration tried to introduce another flag, a vertical triband: blue at the hoist with the stars of the Southern Cross (Crux Australis) as in the Australian flag, then yellow, then green with the white bird of paradise in the fly.
The Southern cross shows that it is a country in the southern hemisphere and can be seen in Papua New Guinea.