@zoe_girl common reasons why they require medical check are below:
•aged 70 years old or older,
•a parent of an Australian and you have a "queued" migration application, and intend to stay for more than 6 months,
•likely to enter a hospital or other health care environment, including nursing homes as a patient, visitor, trainee or employee, or for any purpose,
•likely to enter a classroom environment, including preschool, creche and child care situations,
•known or suspected of having a medical condition, regardless of your length of stay, or
•intending to stay in Australia for more than 3 months, and you have recently visited or lived in a country with a Very High-risk rate of tuberculosis (Tb).
from this site: http://www.australia-migration.com/page/Medical_Q_and_As/12