The passing mark is level 5. Level 4 is the general ICAO standard for flying across international boundaries. CX want people with ICAO level 5 English minimum because often someone with ICAO level 4 English will slip to ICAO 3.x during training.
One of the reasons for level 5 standard is that Cantonese speaking cadets will form small social groups while they are away during flight grading, where they just speak in Cantonese. When they come back their English is 3.x. I recommend that you integrate with other groups and speak in English as much as you can while in Adelaide.
Also this is advertised on Facebook, and it says they are in Hong Kong.
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