my 17 year old non blood cousin would like to continue to live in australia.

I was born in england and came to australia in 1990. I am an australian citizen by decent through my mum. my nan was born in the uk, and came to australia and had my mum. my nan re-married after my grandad died when my mum was 2 years of age. my nan and grandad are now citizens. My grandad (not blood related) has grandchildren from his son who currently live in australia on a visa, not sure what has happened but it is due to work and they are trying to stay in australia as there visa may run out and get sent back to the uk. however, my cousin (not blood related) would like to stay in australia, how can she do this? please help. thank you