Do we need Hong Kong Visa for my wife or we all need to apply for China Visa only?

Hi I am an Australian Permanent Residence and I leave in Melbourne with my wife and little daughter but my wife holds a Nigerian Passport. We both want to travel to China in September for a short holiday about 1 week. We plan to transit in Hong Kong for about 3 days in betweenand when must we apply given that we are traveling in September?