At first I thought that Japanese people pronouncing Qantas as カンタス was an urban legend (the start of "カンタス" matches a vulgar four letter word in English), but I've since found out that it's real, with Qantas' Japanese homepage having "カンタス航空｜ホームページ｜航空券｜旅行情報 - Qantas".
By contrast, there's almost always a "w" style sound when Australians pronounce it. Wiktionary gives an IPA of /ˈkwɒntəs/, and the second sentence of the Japanese edition of Wikipedia's article on the airline mentions how Australians pronounce it with three citations.
Why is Qantas spelt "カンタス" in Japanese?
As far as I can tell, it isn't because it resembles how non-Australian English speakers pronounce the word - in the American movie "Rain Man" they pronounce it with a "w" sound.
"カンタス" could be seen as matching a phonetic spelling of "Qantas", like how English-speakers pronounce "Qatar", but I thought katakana was typically based on how words are actually pronounced.
Alternatively, is it possible that katakana lacks the capability of indicating the "w" sound in this scenario?