According to VicRoads and VicPol, the saliva tests used in Victoria are designed to detect methamphetamine and THC only . The idea is to avoid community backlash from people being busted with prescription meds on board. Apparently, MDMA will cause a positive meth roadside saliva reading, but the lab result will show that it is MDMA, not meth & you will not be charged.
Of course, if you appear "impaired", police can administer an impairment test back at the station (nystagmus, stand on one leg, walk and turn) - if you fail that then a blood test will be taken. If the blood test shows up drugs then you will be charged with DWI - drive while impaired. This is a more serious charge than what you cop for failing the saliva test (XPCD - drive with illicit drug present).