Australia is relatively easy. This because it's a former British colony and thus, it used a defaced British blue ensign, Hawai'i's flag has a bit more complicated of a reason. But to put it simply, Hawai'i was an independent country and didn't want to be drawn into conflict with the United States due to them having the British red design as an unofficial flag, so they purposely added stripes to the flag to appear similar to the United States. Eventually, they were invaded by the United States and became a state and kept the flag. This is because Americans are shit at flags.
Hawaii was protectorate of the British Empire and Australia was part of the British empire (it's still in the Commonwealth), so they've kept that part of their history in their flags :)
Countries like Australia, Hawaii, NewZealand, Canada etc etc etc etc etc were entirely populated and colonized by the British, So obviously these new lands were part of the British Empire, And adopted the British flag. Most of these countries today even consider the Queen of England to be their monarch.
These countries are called the ''Commonwealth'' and are VERY closely alligned with Britain.
Because either they used to belong to England or they still do. Of course, not all British-owned countries had their flag incorporated into their own (Ex: India and Jamaica).
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