Hi Karen
If anything shows up on the x-rays that could possibly suggest exposure to TB at some time, the MOC often asks for a health undertaking. In that situation, the MOC usually also reduces the validity of the meds to 6 months so that a second x-ray can be done, in Australia, soon after the person gets there. This way, if anything sinister is going on the person can be treated quickly and the problem resolved. By reducing the meds' validity-period, the MOC effectively shortens the time that the applicant has in which to enter Australia post visa-grant.
For more info about this, please see here:
http://www.immi.gov.au/media/fact-sheets/22health.htm
Cheers
Gill