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 colonized 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.