Both territories were jointly administered by Australia since 1949, became a self-governing state on December 1st 1973 and independent on September 16th 1975. The western half of the island of New Guinea constitutes the Indonesian province of Irian Jaya . Encyclopaedia Britannica , 1981. If one takes Papua New Guinea as having come into existence in 1949, the only flag that wasn't purely Australian and had the Union Jack in the canton was the flag of the Customs Service. Flag Bulletin number 130 of 1989, which has an article on the flags of eastern New Guinea in general.
Some reports show the tail feathers as brown instead of red.
Among many sources, I believe there is more information in Barraclough and Crampton 1981 . Australian Self Government Current national flag . The chart shows also provincial flags .