The flag of Papua New Guinea is divided by a diagonal running from the upper hoist corner to the lower fly corner. The triangle below it, at the hoist, is black with the Southern Cross picked out in white stars; the triangle above it, at the fly, is red with a golden Raggiana bird-of-paradise.
A schoolgirl's design
Before independence, Papua New Guinea was administered under Australia, and its flag was Australia's. A national competition was held to find a replacement, and the winning entry came from Susan Karike Huhume, a fifteen-year-old student. Her design was adopted on July 1, 1971. Independence followed on September 16, 1975.
National flags usually come out of committees. This one did not.
Black, red, and gold
Black, red, and gold are traditional colors in Papua New Guinea, used in ceremony and in art across a country with many ethnic groups. That mattered for a flag that had to stand for all of them at once.
The Raggiana bird-of-paradise is the national bird and stands for freedom. The Southern Cross fixes the country's place in the southern sky; Pacific peoples navigated by it long before anyone drew it on a flag.
Australia and New Zealand also carry the Southern Cross, but their flags keep colonial references alongside it. Papua New Guinea's has none, and its colors and imagery come from local art traditions instead.
Where it flies
The flag is used at government buildings and national events, flown from sunrise to sunset, raised briskly and lowered slowly. At official ceremonies it stands alongside the flags of other countries. There are no significant variants.
References
[1] Government of Papua New Guinea - Official Publications (https://www.pm.gov.pg/)
[2] Vexillology Association - Southern Hemisphere Flag Discussion (Journal of Vexillology)
[3] Papua New Guinea National Archives - Historical Documents on Independence (http://archives.png.gov.pg/)
[4] Smith, Whitney. Flags Through the Ages and Across the World. McGraw-Hill, 1975.
[5] Flaherty, Mary. Symbols of Papua New Guinea: A Cultural Journey. Heritage Press, 2005.