I spent around 4-5 days in Adelaide in Mar/Apr this year as a tourist. The city is very nice, and it gets several excellent universities. If you drive from the north to south of the city, it takes you only 20 min. Maybe it is far smaller than Tsuen Wan.
So, why not spend some days over there and make an appointment for school visit if you are planning to send your kid over there? There is a direct flight from Hong Kong to Adelaide.
My son study in The Hutchins School in Tasmania, which should be cheaper than major city like Sydney and Melbourne.
經濟証明 is not required from school. However, student visa may required you to show that you have sufficient fee for completion of secondary school. You can PM me for details.